Did they tell you that your breasts are the reason they broke off the relationship with you? It's entirely possible that they could have a different reason and just never expressed it.
Body positivity is hard, but it's possible. Small breasts can be attractive, and often times a personality can overcome an unattractive exterior. Take this from someone who needed this advice 2 years ago, work on your personality before you work on your physical attractiveness.
-KylePD