In fact, if you try to do things too fast, it can back fire on you.
Let's go through some examples so that you understand what I mean.
First, if you have just broken up with your boyfriend, you natural tendency is to think about "how to get my ex boyfriend back fast". Unfortunately, if you have just been through a break up, you will probably feel extremely emotional.
At this point of time, you won't be able to think clearly. Emotions will overcome all logic. If you try to contact your ex boyfriend now, it is easy for you to do and say the wrong thing. This will backfire on you and make the situation worse.
You will do much better if you give yourself some time and space. At the very least, give yourself some time to recover from the break up. It make take a few weeks, it may take you one month, it is alright. There is no need to rush.
Here is a second example.
Let say you have commit a very serious mistake like cheating. In such serious situation, you cannot expect your ex to forgive you immediately. It takes time for him/her to cool down. In fact, his/her trust of you is already broken. As we all know, it takes time to build trust.
If you really want to know how to get my ex boyfriend back fast, just make sure that you don't allow it to backfire on you.