I'm going to be really blunt here though I am sure you won't pay a blind bit of notice, and you'll be juggling fathers around for the next 20 years or more.
You are far too young to tie yourself down any more. You've been a mum since you were 17 ffs. What are you actually going to do with your life? You can't support the children you already have on a PT wage!!
You have an unstable relationship so the idea of launching a baby into that scenario is stupid and irresponsible. You have years ahead of you to have more babies. Why not wait until you are in a settled, happy, long-term relationship. Two years isn't long. I'd be heartbroken if you were my daughter. I have two, both older than you, and instead of making babies while they were barely out of childhood, they've gone to uni, they've lived abroad, they've established careers for themselves. I was with my DH for 5 years before we got married, and we had our first baby 7 years after that. Still managed to have three.
Three children is a LOT. We both had good salaries, owned our own home, and it got more expensive as they got older, not less.
You would be best advised to stick with the medication that is currently working for you and not mess around with it to be pregnant.
Find something else to fill your life up rather than another baby. Do something with it; put yourself in a better position to have a baby later if that's what you want to do.
You've already got a child whose father isn't in the picture. Now that's not your fault, but how sad it is that her sister has her dad and she doesn't. Who's to say your current boyfriend would stick around if you had another baby? Presumably you didn't think the first one was going to fuck off leaving you holding the baby literally?
Just don't.