How do I cope with being a stepson?

My mom left my dad many years ago and I don't really have much contact with her, she lives in another state and she was abusive when I knew her when I was younger. For many years I was basically brought up by a single dad. Recently he got married. I initially got along really well with my stepmom but ever since she had a baby, I have definitely felt like baggage. She's still nice and all most of the time, but I feel like a second-cl citizen in my own home. She doesn't really care about what I'm doing - I feel really guilty because I often feel like I'm invading her and my dad's space. She doesn't like my girlfriend or any of my friends coming over and gets cranky when I want to, saying she just wants to spend time with my dad etc. The thing is, it's not her fault, I know she's nice etc, but I really do feel like an obligation from a one-night stand. I get very sad/lonely sometimes because I don't really have a 'mom'. What are some coping mechanisms for when I feel really down?