If you intend to continue the pregnancy and have a baby, any drugs you take are going to affect the baby. You need to focus on the health of the baby more than how much weight you will gain. The weight you put on is necessary for the baby's development. But your husband should know about it, and just because this other chick is preggers, too, shouldn't be something that keeps you from telling him.
If you do not want to bring the baby to term, an abortion would still be a safe, simple, and low risk procedure at this point (before 12 weeks).
You seem to suffer from low self-esteem. You've gone along with an arrangement you didn't really want, defer to other people whom you place above yourself as more important, and are afraid to speak up for what you want. I think you need to be honest with yourself and your husband, and perhaps look into getting some therapy to learn ways to boost your self-esteem.