They'll never believe it.

Maybe it's some unhealthy yearning to be the protagonist in a piece of especially supernatural character-driven YA fiction, but I have always harboured the desire to be able to travel back in time and revisit Past Me, remind her of the people she's only just met or show her photos of people she has yet to even come across, and tell her the stories. After all, ninety-nine per cent of the time first impressions count for nothing - yet one per cent of the time, they are everything. I'd love to show Past Me if her instincts were correct; I'd love to see the look on her face when she's told certain things.

Let's start slow. 
The guy with the cranberry-coloured hair who you're drunkenly chatting to at the SU, he becomes your best friend, you go on an epic holiday with his family, AND in third year he falls on his feet and starts dating the first girl who spoke to you in your first Drama lecture (the super-cute one with those eyes). The guy you're seeing come the end of Freshers fortnight sure can be a complete fool with the ladies, and you'll hate him for a little while, but he'll grow up pretty damn well sooner or later and you'll get along just fine. The creative genius chick in your poetry seminar will be one of your dearest friends before long, back home she lives a fifteen minute drive away and she'll inspire you constantly and consistently. The lovely lecturer you have a massive creative crush on, someday he'll be messed about by the uni; you and your other course mates will help him get a permanent position secured once and for all. That rather beautiful perfectly ginger American gal in your Drama lectures for one semester will earn her title as an Honourary Brit, and despite being across an ocean most of the time, she'll always be there for you.

Now, some pleasant surprises... The girl you converse with over Twitter about your mutual love for Joshua Radin? Someday you'll be sitting side by side in the most picturesque chapel in Islington, watching your favourite artist onstage bewitching the crowd. Oh, and also you'll meet Joshua himself after all those years lying on your single bed playing his albums on repeat as you contemplate your existence and fret about the boy in your English class. You'll hug him, he'll write down your favourite lyric in Sharpie pen (a pen which you keep) and you'll get it tattooed. 
The crazy chick sitting outside your Sociology classroom chatting about hair dyeing with you (after you told her your bright orange hair was a horrible hair henna-related accident, then she reassured you it looks rad); she'll be a constant for some years, always reachable online and always up for a tipple, then suddenly you'll join the team on her exciting new project and she'll be your wicked-perf editor. 
The guy in the Single Honours Drama clan with the long blonde hair, the one you met while drunkenly scoffing chips from the food hatch on a night out, will become a good friend and a fantastic drinking buddy. You'll be in plays and performances together a fair bit over the next couple of years, then discuss moving to London with other grads.
That Fresher chick on the Ultimate team who you so desperately want to like you, despite hearing that she definitely doesn't - give it a year, she'll be your best friend. For real. The best of the best.

Yes, some things are upsetting.
The girl you've made friends with during Freshers week, the one you take all those webcam selfies with, yeah that friendship won't last. She'll shag a guy, stake her claim to him, then he'll stupidly (and completely independently) take a shine to you, and she'll make it all your fault. Don't worry, your lecturer will defend you and shut her up when she screams at you in the middle of a Drama seminar. 
Your second year house mates will be challenging. The 'lads' will relentlessly take the piss because they're both strapped in with long-term relationships and you're bringing guys (as in, TWO different guys over the course of one year, you slut) back to your box room for nightcaps. The other girl will turn up her nose at most of your guilty pleasures and best intentions, but when it comes down to it, she's a decent friend. You two have plenty of nice moments drinking tea and watching Gilmore Girls. 
Don't push it too much with your Ultimate team mates. If they like you, if they're wanting to be friends, they'll reach out. Stop smothering them and scaring them off with your mad chatter, just because you're scared they won't like you right away.
Your college besties, they hurt you like no stupid guy ever could.
One last thing. Those new third year house mates of yours. You cook with them and drink with them; you'll be discussing the politics of sexuality one minute and the best essay-writing techniques the next; they support you better than anyone, most of the time. By Christmas, you'll never want to be in that house. The damage will be irreparable. You'll be crashing at friends' or your other half's (we'll get to that in a minute) or even setting up camp in the library until the small hours even though you only have a couple hundred words to write... You hate being at home. But you find home elsewhere, don't worry.

Some revelations are uplifting.
Who is that boy in two classes with you at college? He's a laugh, apparently. He entertains in Drama and has his head down in French. Admit it, those yellow-blonde inconsistent highlights through his brown hair are somewhat endearing. He's your soulmate - and he'll prove to be an invaluable friend to you for years to come. Don't try to live without him, you can't.
Your two playgroup besties who both live with their families across the road from you will be lifelong friends; one day suddenly you'll all be twenty-one and having a cup of tea together still loving each other's company and totally at ease together.
And that guy your course mate and colleague introduces you to, with the colourful tattoos down one arm and black work down the other - the boy you shyly say hi to, and he's shy right back - he'll make you the happiest you've ever been. One night you'll come home from work and he'll be waiting with a cup of green tea, plus biscuits and a big smile. He genuinely cares about you, and you can barely believe it. You did good there.

I'd also want to tell my past self about the experiences she'll have in the near future - but I wouldn't want to terrify her beyond belief. I think she'd get a lot of excitement out of seeing photos of future friends/enemies/something-mores. She may somewhat perversely love the fact that someday she'll get her heart smashed to pieces which are then scattered around for all to see, that's just an occupational hazard of a hopeless romantic, and at least it means that she has a mad mess of thrilling feelings and epic drama headed her way... She could probably even deal with the fact that she has a life-changing medical revelation come the end of her degree. I wouldn't tell her that she gets an Upper Second Class, or that she gets Firsts in certain essays, or that her old school invite her back to make speeches and she receives insane endless applause and positive feedback after she speaks - she couldn't handle that pressure. It's better as a surprise. I wouldn't tell her that she moves back to the family home after uni - because even though it's due to the medical drama, and even though it's for practical rent-free saving-up purposes, and even though it's nice and comforting for a while... She might be disheartened, and think of it as a failure. I may let her know that someday she'll suffer from depression, because maybe then she'll realise what it is earlier, maybe then she'll get something done about it earlier, maybe she'll read up about it and be properly prepared... That is, if it's something you can really prepare yourself for. 
I'd definitely tell her she becomes a barista; she gets four tattoos and ten piercings with intentions to get more; she develops a love-hate relationship with the bottle; she figures out how to make time for writing and reading recreationally; a fella finally gives her hers; she finds a real love for and has fun playing an actual sport (Ultimate); she eventually finds her way around the London Underground; she meets a writing hero of hers, John Green, and he wishes her luck in life; she finds a perfect way to express herself through blogging. 
She does alright. And the people in her life, the ones she never thought she'd befriend and the friends she never thought she'd lose, they get her there.