In need of some advice on my situation

I hate feeling like a third wheel in this life. it is hard to express what is wrong without worrying that my partner will more than likely get defensive and angry. I hate it so much because what I want in a relationship is not what she wants so I will never get it so I have to just grit and bare it in hopes that things my work out. Unfortunately doing so makes us both think that the other is bothered which is likely true but as much as I want to share it I know that she will not. Why do I have a empty feeling in my chest where my heart is? This is every once in a while thankfully because it makes me feel alone, lost, without a singe person to confide in....mom I wish you were still alive just so I can have you to talk to even tho it was a rare occasion that I would call, and right now I need that call so I don't *** things up once more and actually be once again alone in this messed up world. I love this woman that is beside me and want to marry her......but alas we lack the biggest things in all relationships and that is communication. Beleive me when I say this it drives me insane to know that she worries about her babies dad so much more and is by his side daily, I get it they have had a long relationship of 14 years but wow I think she is dating me.....but I am not really sure if I can take much more of the lack of attention that I think I deserve. I give her the world if I could but why should I? All I see is her going to him every time she wants something or when he is hurting inside so I feel I an not the right one for her and he should ask for another chance because I see what I see and it makes me want to just back off and walk that sad an lonely path again. Truth be told if I ever bring it up on what I see I better be prepared to argue and walk away before I do something I know I will regret because I don't like having a feeling in my heart that does not belong. She keeps herself devided and distant from the man she loves all because of what tracy is doing to him because he did it to her so she does sympathize but in return it puts me into a position that is super uncomfortable and have to bite my tongue because she will yet again go into argue mode or shutdown mode because of what I say and I don't know what to do any advice would help me come to the right decision. Keep in mind we both ae drug addicts plus she is an alcoholic on top and shows no sign of quitting either of them but I don't want to do drugs any more I want to make better decisions sober.