baby, you should get out of this relationship. It isn't healthy.

Guys of quite a few years older go for school age girls for mainly two reasons, sex and control. Even the sex reason is truly about control as in who gets to say how much sex will be had. The guy, being more experienced, gets his way. They can be very nice and charming, but they count on the girl's ignorance about what her place in the relationship should be. Being in control, they can slowly work up to fulfilling their fantasies.

Tie this in with you being beaten by your father, abandoned by your mother and never fitting in with your adoptive parents and I think you are unconsciously looking for a strong adult figure who will give you unconditional love and be a strict disciplinarian (a parent, not a best friend). To some degree you probably equate sex with love so your desire for love equates to a desire for sex. Your bf is working on controlling you, so limiting the sex when he doesn't have as strong of a sex drive or has very strong control over his sex drive, is one of the ways. He is also using you to take care of his child.

You should get out of this relationship and finish up your GED and then continue by going to college. A job may be helpful to do this and to you as you will see your self worth over time and your self esteem will grow. The job may slow down your studies but it will give you independence which is very important.