Venting Over My Relationship

I’ve been with my boyfriend for 3 years and he has a daughter the age of 9. I’m 22 and he’s 34,yes I know there is an age gap lol. But anyways it’s been very tough sometimes and I feel especially now with Covid. I love my boyfriend and his daughter with all my heart but sometimes I feel like I can’t handle being here anymore with them. I moved in about 8 months ago. We got here a iPhone 11 since she was doing good in school and she was changing her behavior. She’s always had a very bad attitude when she doesn’t get her or if she doesn’t like something. We tell her to eat and she refuses, homework time nope don’t think so, give us your phone you’re grounded and that’s when it all goes down hill. My boyfriend and the mother of his daughter have split custody of her. Ever since I’ve been with him he would have her for a week and then she will. I’ve known her as a woman bouncing from guy to guy and once Bella came up to me once and was like how come you don’t have 2 boyfriends like my mom. My mom uses guys just to get money from them. My boyfriend gives Bella every single little thing and he spends so much time with her and sometimes it’s like Bella doesn’t care. When she gets mad she’s like I rather be suffering with my mom and bounce house to house and she has a car she gets from her friends and so on. One time Bella came and didn’t want to sleep it was 12 in the morning she was playing with her phone and she had school next day. Everything went so crazy she screaming, crying, kicking me and telling me at least I’m not poor like how you were when you were little at least I have a house. She’s done that to me 3 times. Always says I rather be with my mom. Her mom practically abandoned her a year ago and she just left her here and doesn’t take her for the time she should and only sees her whenever she wants or calls her like once or twice a week. She says why do I have to look for her she should look for me. She told my boyfriend I’m trying to figure out my life and she till bouncing house to house said I need like a 1 month for I could find somewhere to live. Well it’s been a year and 5 months and she hasn’t gotten her shit together. It’s been very tough to deal with Bella and sometimes I think she has a behavior disorder. I tell myself like why am I here I’m too young to deal with this. I don’t know what to do and it breaks my heart having these thoughts. Bella tells me sometimes I don’t like you and I planned with my cousin to have you break up with my dad. I treat this girl as if she was my own so it hurts her to say that even when she says my dad doesn’t pay attention to me because of you. I give my boyfriend so much space for them to have because I know she needs it. She told both of us that she doesn’t want her dad to be in a relationship with anyone because she wants him all to her self and I’m like sweetie your dad is an adult and he can be with whoever he wants you can’t stop that from him no matter what he is your dad so when he’s with someone they have to understand you come first. What hurts me the most is that I had to do the biggest thing in my life is to move out out of my moms and my nephews being there. It hurts me knowing I don’t see them as often or that I sometimes choose to spend time with Bella when she needs it. I miss my boys so much I cry when I think about them but I knew I had to move out some day and build a life of my own. I’m sorry this is all over the place I just kept writing as what would come through my mind. ——Forgot to mention that he does all the parenting obviously because it’s his daughter and I just step in when he ask me to. I don’t want her to feel like I’m trying to take over her mom but she needs to know that I will try to teach her morals since I’m with him.