So, you're 18 now. That means you'll be starting university soon. University won't be how you imagine it to be: you won't meet 'the one' but you will meet some amazing friends who you'll be friends with for life. You won't enjoy clubbing and that's fine as you'll meet others who feel the same way. And even though you're studying an academic subject, you'll always have that niggling feeling that you're not doing quite what you'd like to be. And that's ok too - things will work out in the end; life's funny like that.
You've come a long way from being the girl who grew up in Scotland and then the midlands of England. Throwing yourself into the heart of London is an achievement in itself and something that you never had second thoughts about: keep this fearlessness as it'll definitely come in handy in the future.
You'll change as a person (for the better) and it'll be a gradual change. They call it maturing and you'll do plenty of that in your three years at university. You'll play more than you worked but that's ok too - you'll get the chance to do university again where you can make up for this.
A lot of unplanned things will happen in the next seven years and although some of them will knock you back a little, you always find a way to pick yourself up. That's because you have amazing friends and family who are there for you every step of the way. Life is unpredictable and you can plan all you like and overthink things all you like, it will always stay this way.
You think you'll have your life sorted by 25 and let me tell you this now: you won't but that's ok. And you'll meet many others who won't have everything sorted by 25 too so you won't be the only one. You'll be just over halfway through medschool but you're doing something you love and that's the one thing that matters the most.
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Have you ever done this on your own blog or in your journal? I use futureme.org to write letters to my future self and it's surprising to read back to see what was important to me a few years ago. Priorities change and I've also changed a lot as a person if those letters are anything to go by!