Wow, you must be totally exhausted with it all and your friend is being selfish. You are obviously a very caring person but they are draining you, which is not ok (you need to care about your own MH, not just theirs).
It's hard to suddenly stop doing something, but you could put a time limit on it by telling them that you are not going to write any more essays for them past the current module, because you need to focus on your own studies and need to start preparing for the first year exams. They will also need to take responsibility for their own work from now on because you cannot sit the exams for them - if they don't wake up to the fact it is THEIR degree, not yours, they will fail the exams and have to leave Uni anyway. So think of it as being cruel to be kind, if they start trying to guilt trip you into carrying on in the same way.