Often we would want to win an ex-girlfriend back but it will not be easy, we will have to earn her in order to get her back. This is a little different from just getting a new girlfriend as an ex already knows your better chat up lines and witty rebuts and so you will be playing a whole new ballgame with different rules but you can still win if you really want to.
Today there are websites like www.getherbackguide.com which can advise you in this regard but they can only offer advice, it is up to you to take that advice and make it work. Your first thought will probably beg her to come back but think again as begging will make you look weak in her eyes and so if she did come back, it would probably be through a feeling of pity. Groveling, of course, will always be necessary to some degree or other but stop it short of begging for lasting results.
In order to know how to win her back, you will first have to learn the reason why she broke up with you if you do not already know. She is not likely to take you back if you still have the same fault or faults she saw in you before. Remember this though; even if you do not see it as a fault, it is what she sees it as which matters and so be humble and accept that perhaps it may be wrong on some level. Having discovered what you need to change about yourself, you will have to implement that change as just telling her you to plan to change will probably not work: show her something to convince her that the change is already underway.
Another mistake that guys often make is to remind an ex of all the good times they had together and although this may initially seem like a good plan, evidence shows that more often than not, it also reminds the girl of the bad times, reminding her why she left you. You should, therefore, concentrate on the future and speak only of the good times you could have in the future. Basically do not dwell on the past, show some initiative and look to the future and impress her with what you visualize for the two of you. Of course though, if the relationship is going to last, she may expect you to make good on at least some of those dreams and so keep them realistic, perhaps a romantic dinner or secluded picnic, nothing which you could not make happen with a little effort. The sooner she sees you are serious about the future, the sooner she is likely to take you seriously and give you a second chance. So, perhaps you should have one dream already worked out in practical terms with perhaps a date and venue for the event, something solid she catches hold of and takes you back with.