Why is it that when the devil child (boyfriends) sd10 is not around life is peaceful?

I'm sorry but I can not handle even hearing her name. I just can't do this anymore. I love my bf but I don't want to be a step mom or ? I thought I could try and did try but not anymore. Between the devil and her mother I find myself doing and being this person I don't recognize.
I use to be so strong, so fun, I laughed all the time, I did anyway. Now its rare. I walk around with a frown. I don't even have the will to fight about all that is going on or gone on. I worry more about getting shot down and feeling even worse than I already do.
I have thought about going to a therapist, and that's just sad. I can not believe that I allowed someone to bring me so far down into the trenches I hate myself.
I just now need to find a way to tell him. A part of me wants to curse and scream and tell him what I really think and then the other just wants it to all just drive away.
But what I really want, is for him to see that I am deserving of love and he allowed his ex and sd10 to screw it up.

I'm broken hearted and don't want to be alone starting over again. But I know that I can use this as a lesson. Next time I will not make someone a priority when they only make me an option.

now what...