Even though there is no surefire way to win your ex boyfriend back, there are some important things that you can do right now to maximize your chances of rectifying the situation and patching things up with him. Here is what you can do to work towards a solution with your boyfriend and hopefully start over with an even stronger relationship.
Offer a sincere apology for anything you've done that may have caused or contributed to the break up. This is the first step in repairing a damaged relationship and it cannot be understated. You need to make sure that your apology comes from the heart and is completely sincere or your boyfriend may sense that you are being untruthful or fake and he will immediately dismiss any ideas of taking you back.
Explain what you intend to do differently. It's important for him to know how you intend to change in order to make the relationship better. There are two sides to every relationship, but if you are the one trying to get him back you may need to offer your ideas about what you will do different first.
Respect his requests for space and time. One of the worst things that you can do to ruin your chances of getting your boyfriend back is bother or harass him. Constantly calling or texting will only make him want you less and less. If he wants to keep talking then that's great, but if he makes a request for space or time you need to make sure that you honor it.
Don't act jealous or ask questions about other girls. Even though this may be an immediate temptation it will make you appear desperate or controlling and will only irritate your ex boyfriend even more. If you find it absolutely necessary to bring up another girl you need to do so in an appropriate, civil, and conversational manner.
Avoid making demands or trying to impose ultimatums. These tactics don't do anything to help the situation and make you look like a vindictive, angry person. No matter how angry you might be you need to refrain from this type of behavior.
Learn to let go if it seems like reconciliation isn't possible after putting forth your best effort. Sometimes a certain amount of time needs to pass before it's possible to repair a relationship and sometimes no amount of time will make a difference. Either way, it's much healthier for you to move on if your boyfriend doesn't want to get back together after tried your hardest. In this instance, you may be much better served to go on with your life and look for a new person.