We all find ourselves in situations where we feel helpless and devastated. Post-breakups, post-fights, and all mood distorting experiences that put us so low that it is very hard to think apart from our clouded heads. But note this point, it is 'tough', not 'impossible'.
Ofcourse, everyone becomes a pessimist about themselves on various occasions, but ask yourself once, 'Is it really worth putting yourself down?'. You are supposed to be the last person who would put yourself down, no matter how many people have done you the favour already.
You had a breakup, fine, it's a bad phase and real tough nights are going to pass by. But, it's going to pass, not stay. You need to know that there are various phases ahead and so much life you need to see, experience and live. The person who so willingly let you go, do you think he's going to be affected when he hears you doing something bad to yourself? Do you think it would be more than anything to him rather than talking to various people about it? He's probably going to make a gossip out of it to make himself look very 'influential' in your life that you just fell for him so much, and that's it! That's it. He will forget about the gossip eventually, the same way he forgot about you, and so will all his anonymous friends.