Jasper fucked up. Jasper was abusive and unhealthy for Lapis. It showed in their fusion and in their interactions. Jasper had issues and she would have destroyed Lapis mentally and emotionally had they stayed together. Don't you fucking dare blame Lapis
This is why I really love the episode "Joy Ride". The Cool Kids listen to Steven's problems and they take him out for some fun. It was really nice to see because he's so young and they just care about him so much and they protect him and it's so cute.