First of all, most, if not all women, have asymmetrical or uneven breasts in varying degrees - your problem is more common than you realise. Just like most people's feet are different sizes, or hands even. Every time I get fitted for shoes in a store they tell me one of my feet is a half size bigger than the other - I was really surprised the first time I was told this because I didn't realise.
The only way of correcting asymmetrical breasts is by surgery. I'm not sure where you live or what your health system is like but I think it's worth seeing a doctor to put your mind at ease and discuss your options.
With regard to the guys walking out or making you feel bad - firstly, they sound really young and immature and not worth feeling bad over. Secondly, I don't want to tell you how to live your life, but with a regular boyfriend who cares about you, I can tell you things would be different. People who care about each other shouldn't make each other feel bad about their appearance, whereas more casual hookups are far more shallow about their feelings.