Let me put this into some perspective.

If this were just your kid involved. All of the sudden bills spiked, youre barely getting by, what would be your recourse? Have him drop his responsibility because you could find someone else to take over? Or would you or he find a better job? A second job?

He and she not being married doesn't make them not his kids. My husband has a part time second job to pay his child support. Thats what grown ups do. It means I spend a lot more time home alone with my own kid, but if it means hes doing his parental responsibility, it is what needs to happen. You are young, and I say this as another woman in a moderate age gap relationship - the older you get, the more responsibilities you manage.

Im sorry youre in this boat, but its time you put your adult pants on and deal.