Before the Breakup
Breakups are not usually unexpected.
Rather, there is a run-up period before the final, definitive dissolution of the relationship. During this stage of the breakup you will likely find yourself fighting with your significant other a lot, not speaking to him at all and generally feeling uneasy. Deep down, you will know that the relationship isn't right, and it is important that you confront this and actually break up as soon as possible.
Once you actually break up, there will be a stage in which you mourn the end of the relationship and miss your ex.
This is the stage where you will start to forget how bad it was at the end and also miss being around someone who knows you so well. During this stage, you'll feel sad, vulnerable and generally miserable. Indeed, you will probably feel worse during this stage than you did during the pre-breakup stage, but this stage is much shorter and marks the beginning of the healing process.
Once you have some time between yourself and the end of the relationship, the mourning will turn to anger.
During this period, you'll often find it difficult to concentrate as you think about the things she did wrong, the time you wasted and the other people you could have been dating if it wasn't for her. This anger will eventually turn to bitterness, in which you are angry at her but also angry at yourself for wasting your time.
The final stage is the nicest, but it is also the hardest to pinpoint.
This is because it is the end of something rather than the beginning. As you move through the breakup, you'll think about your ex less and less every day. Eventually, you will realize that you haven't thought about him in weeks -- and when you do, it is without any anger, regret or sadness. You won't notice this stage coming on, but rather will notice that you've been in it for weeks. When you realize this, you'll know you're ready to fall in love again.