If you have been a jerk and have been treating your ex girlfriend very badly which finally lead to the breakup, then you deserve it. If you still think that "i want my girlfriend back", then I do not think that you deserve another chance. However, if you know that you are the real source of problem, then you should first figure out what is the problem with you and resolve it first before you can win back her heart.
Below are some tips that you can follow if you know that you are the real problem and insisting "i want my girlfriend back".
Women Should Be Respected
You should first change your attitude and treat women like human and not your property. Only if you treat everyone with respect, including your girlfriend of course, then only you might have the chance to win her back. You need to be treating people well and give the respect they deserve before people can respect you.
Give Proper Treatment To Your Woman
Treat your girlfriend well. Behave like a gentleman, but not pretending, as faking it will never be good as your girlfriend can sense it very easily. You should treat her like a princess and if you are able to do this sincerely, the possibility of "i want my girlfriend back" will be higher.
Be Honest And Let Her Know You Were A Real Jerk
There is nothing to be embarrassed about if you really still love her and want her back. After all, you have started everything for ruining the relationship for being a jerk, so you need to be sincere and admit to her that you are a real pain in the ass previously and are willing to change.
If your girlfriend wants some space and need some time to think about the relationship, give her the space and the time to cool down. Respect her decision and not be a jerk once again by pushing her. This way, she has some space to breathe. Behaving this way instead like a jerk is good because such behavior may change your girlfriend's attitude towards you and hence the chances of "i want my girlfriend back" will be higher.
Do Not Just Talk, Show Some Actions
Don't just say that you will change. Show her with actions. Prove it to her that you have all the things she wants to get back together again. If you are even able to convince her friends that you have really changed, then your chances of getting back together again will be higher. Actions tell everything whether you really changed.