This sort of mental struggle is usually internal, not unique by any means, and you probably aren't even aware that it's happening to you.
Without even realizing it, you are probably punishing yourself.
What is likely to have happened over the last few weeks is that you have spent most of your time alone, and your whole attitude has become negative.
In order to avoid a deep depression after a breakup, the very first thing you must do is to understand that you deserve the right to be happy.
You have every right to be happy.
Everyone deserves that, you included.
What you should do next is to identify anything you do that has a negative effect somewhere down the line.
To make yourself feel better, when you over indulge in a particular thing it usually makes your depression worse after a few days.
For instance, if you eat loads of chocolate because it makes you feel better, you are only letting yourself in for more depression once the extra weight you have put on begins to show.
Depression is a horrible thing - all you want is to be left alone with your sadness and misery.
What you have to do, though you don't want to, is to get involved with things that have a positive effect.
Go and get your hair done.
Go out with your friends.
You probably think you won't have a good time, but force yourself to go anyway.
Get out and about.
The reason for this is that for a few brief instances you will forget how miserable you feel.
The more you do it, the more you will get used to the good feelings, and the more of those you get, the more you will want, until your life is back to normal again.
Do all of the things that make you feel happy and that you really enjoy.
Stay away from the things that only give a short term relief such as drinking, gambling, eating too much or anything else that will come back and bite you further down the line if you over indulge.
Only then will you be mentally strong enough to start to work out a plan to get your ex back.