How should a stepchild feel being part of a new family?

I feel really unloved by my family. My parents loathe each other. They've both remarried and got more children. I feel totally unloved because they both fight over NOT having to look after me (I was an accident pregnancy). I've overheard them: 'no you take Dave, you're the one who wants him more, just let me live my f!@#$%g life' etc etc. I'm no longer on speaking terms with my mom for a variety of reasons. I now live with my dad & stepmom full-time. However I feel like I am constantly intruding on their space - my stepmom's never had kids before and wants a new family and I really do feel like I'm just baggage. Particularly whenever I want to do anything with my dad on the weekend and he says yes, I then overhear them fighting because my stepmom wants to spend every weekend with him and the baby together - I've tried at least 15 different times to do things with my dad (play sport together, watch football together, take me for driving lessons, go to restaurants etc) and he says yes, but my stepmom puts her foot down because time with her and the baby is more important than me. I try to help around the house, I get straight As, I never ask for much - I just feel completely useless sometimes. How should I behave/feel as a stepchild to fit in better with my new family?