I dont think there is anything wrong or selfish with not wanting kids and breaking up over this. However, if you do decide to give it a go I think it would be a very good idea to sit down with bf and negotiate about what responsibilities you are to have involving them. Personally, I wouldnt want to have to discipline anyone elses kids. Secondly, while you see all the negatives of the situation and theres plenty of people here who have negative histories in similar situations I know there are positives too. You say you can relate to the 11 year old emotionally, that doesnt only have to be a trigger. That could be a point of bonding. You could even try to symbolically try to bond with her how you wish you had been treated at her age. You also mentioned you were in therapy, if you do decide to try this out, spend time in therapy about how you will cope healthily in situations that you know will come up. If you feel triggered by an 11 year old attention seeking, how could you handle it in a healthy way? Stuff like that will go a long way IMO of letting you enjoy the new situation.

I dated a woman with kids and to be honest, there was stressful moments but also there was some amazing moments where I grew to understand myself better and actually had fun being with them. Ever see a new toy on tv and go oh man I wish they had that when I was little, well now with kids as an excuse you can!

Whatever way you end up going Im sure will be right for you. If you do stay though try to prepare yourself and your bf for whats coming. If you dont, you dont.