You feel like you were with them for however long, but it might as well have just been two seconds, because you feel like they never cared about you at all anyway. You feel really insecure; like you weren't good enough for them, even if you left them.
Breakups suck, and they hurt a lot more than how they look. Breakups can cause people to become stressed, distressed, depressed, and even in some cases suicidal.
Don't let a breakup play on your mind too much; it's really not good for you. I just broke up with my ex and the only way to cope is to just never think about her anymore. I'll be here if you need any further advice.