I had a baby at 20, it is hard, really hard. You think that it will be fine and you will cope and it will all be lovely, and yes, you might, but you'll be skint all the time, wishing you could buy the things everyone else has for their child/home/self, and being unable to. You might feel stuck at home with the baby/young child (I did, and I never thought I would do :() and then when you get a job, or a place on a study course, you'll look at everyone else your age living their carefree lives and feel sad/resentful that you don't have that. Relationships become complicated because there's an extra person involved to consider, and you can't just carry on with a life of partying, drinking, casual sex - and okay that might be destructive but it's damn fun 
and if you haven't completely got it out of your system then you'll probably feel nostalgic for all that too. It's not really something you can pick back up when your child is in their teens.
Friends become hard to make because there aren't that many people who have children at the age you are, so people your age are off out partying and don't take into account that you need time to find a babysitter, or don't understand that you might be able to come out but will have to be back by 12 to relieve the babysitter. You might be lucky enough to have someone who will happily take your child overnight but they probably won't want to do it regularly, so your friends who go out several times a week will bond in their group and you'll end up feeling pushed out. Mothers you meet at mum and baby groups might be nice people but they're often a little patronising or look down on you for being young. You will make friends, but they're hard to find and take a while. You will feel sad that your child doesn't have a Dad around... even if you think this doesn't matter now, I've come to realise that it does (and DS has a substitute father in the form of my DP now) and it's just utterly exhausting dealing with a child and all the housework, bills, food shopping etc by yourself.
I don't want to tell you what to do, I just wanted to tell you the things that I didn't think about when I was pregnant at 19. Sometimes it's easy to have a bit of a rosy picture or perhaps a blurry one - I knew it would be hard but I didn't know what "hard" would entail, not really. I hope this helps you make a more informed decision either way.
As an aside whatever you decide to do, it might be worth booking an appointment with your GP or practice nurse to discuss contraception options once you're no longer pregnant. It might be that something like a mirena coil or the implant is good for you, both of these stop periods completely and you don't have to remember to take them. They should be happy to explain to you how the different forms of contraception work as well, I understand not trusting something that you don't know how it works, but it wouldn't take long to ask and then perhaps you would feel more confident with it.