Abortion, is it an option?

I’m 28 with 3 children. 2 from a previous relationship (marriage) aged 7 and 5. And I have a 7 month old with my current boyfriend who has two children from a previous relationship as well. We found out a week ago that I’m pregnant again. I missed a birth control pill and he didn’t pull out BUT I took a plan B less than 24 hours later. I still got pregnant. I’m beside myself. I’m sick really. I feel like there’s every reason not to have another baby. I have a 7 month old. We took the steps to prevent this. I want to enjoy my current children. But this wouldn’t be my first abortion. I’ve gone through this before. I know how empty I’m going to feel afterwards. I know that I will regret it. I already do and I haven’t even made a decision. We live in a two bedroom apartment with three children. We were supposed to be buying a house next year but this is going to set us back. It’s so hard to look at my 7 month old and think that I have another baby, another littler person just like him growing inside me. I don’t know what to do. There is every reason not to have this baby. But I want to.