You're just finishing school, don't be scared of what's to come. You're going to make friends, lose friends, make good decisions and bad decisions... C'est la vie! Stop dwelling on the future and focus on the present, worrying will not help at all!
So maybe at twenty you're not a published author, a celebrity or a professional travel blogger but think of it this way... You have years and years to grow into who you're going to be. If you want something just go for it, stop looking at all the negatives. You're going to go through your fair share of ups and downs but after a down there's always an up so hold on and don't be a miserable cow.
For goodness sake, stop dying your hair. It's been many colours and in the end you're going to chop it all off and have to wait through the awkward pixie stage. It's boyish and you'll look like Sam, stay away from the bleach and let it all fade out!
Speaking of which, don't make impulse decisions when it comes to cut and colour. Whilst hair grows back, you're hair is too frizzy to end up looking like the picture you show to the hairdresser. You won't like a great number of your impulse choices when it comes to your hair, just don't complain when it goes wrong because... It will.
You're going to see some amazing things, you're going to travel and you're going to make memories that will make you look back and smile fondly. Don't be afraid of planes, you're tough enough to handle long haul flights on your own. Even if you do get support from the random Asian couple who practically adopt you on your first solo flight...
Oh! Also, you're irrational fear of boats will also clear up. You'll have a fab time in Tenerife on the whale watching trip so don't overthink things. You'll see what I mean when you get to that part, believe me!
Maybe you should stick to something for longer than a year, maybe I'm saying that because I have hindsight but you know that I'm right. History, Journalism, Social Media... Yeah, you have a long way to go.
Here's what you look like now you're twenty, as you can see, you've switched up your style a bit and you're looking a little more natural... It's taken a while. All I can say is just stop being so self conscious, the thing that you think are important just really aren't.
This letter of advice is a bit all over the place but since I'm writing this specifically for you I assume that you understand that it's just how I am. Take some of my advice and remember that you have time to grow (not literally, you're still barely 5'4).