Am I alone?

My boyfriend has custody of his niece and things were great at first. I fell in love with this little girl. She was my little sidekick. Let's just say through all the jealousy she has towards my boyfriend and I being together and just all the things she does to lash out at me, I look at her as the spawn of satan now and I just wish her mother would get her shit together and take her back because I'm literally done with this. I have yet to meet someone in my shoes because let's be honest, the circumstances are weird. But yeah. Just had to vent because it's getting really difficult. I try so hard to stay positive and build a positive and healthy relationship with her but it's not working and I'm about to say fuck it. I feel so bad for my boyfriend to be in the middle of it because I can tell he's at his wits end as well.