You complain you feel like you have half a family, breaking up will certainly guarantee it. I guess you think you would feel some sense of control over the whole situation, which you don't feel now. You're feeling like you and she are rivals, and whoever gets their plans in first "wins." Do you assume that if you broke it off with this guy, you would meet someone new who doesn't have kids already and would want you and your kids as his family? How would you feel having to juggle family commitments so your kids can see their dad on some weekends? You'd be as out in the cold as his first ex.
It might be better to see a counselor together and tell your boyfriend (with the counselor there) just how left out you feel when he makes plans to see his other kids without consulting you first to see if you have any particularly special thing planned for the weekend, such as this event you had in mind in commemoration of your dead cousin. It would help so much if the two of you talked all of this over with counseling help.
Should we break up I want to I want to have a family an they way things are going I'm gonna have half a family and me n mines will always be left out at some point