Fed up!

My husband and I are married for almost 4 years. When we were dating his ex girlfriend was already pregnant with his daughter. I decided to stick around and offer all the support I can cause he ask me to, promising we won't have problems. I took care of his daughter one entire year until her mom decided to take her back. Ok ! I let her go after all she isn't mine,I built a very strong relationship with their daughter which today isn't that way anymore.Now that she took the baby back we're sharing custody 15 days, everything I buy for her daughter she doesn't take care of it or it always gets lost , she doesn't buy anything for her child we have to. What hurts me the most is that it was never my intention to take her child from her,she's putting the little girl against me, the child no longer calls me aunt or obeys me and when I talk to my husband about it he's says I'm being too sensitive, her mom comes into my yard without my permission and since I don't feel comfortable about it I spoke to my husband yet he still doesn't defend me nor give me my ground for him I'm always overreacting and being sensitive hence every 15days I have to feed,cloth,bath,drop off to school,pick up and educate their child. I'm getting fedup of this situation and I don't know what to do any advice will be appreciated.

( I don't have kids of my own . I'm a university student)