I think girls put more thought into whether they want to close the chapter on someone or not, but when they have closed it, it's not going to be reopened.
As a guy you just have to respect and accept this as these are the type of situations where you risk losing some dignity by chasing someone. When you make attempts to rekindle an old flame and you just get repeated rebuttals it is quite painful to have it slowly dawn on you that she is just not interested.
Always best to deal with getting over her on your own or with your own friends and don't make any contact with her or let her know how you're feeling. Guys often make the mistake of thinking that all girls want a guy with strong feelings for her and if you communicate this to her properly she will want you back, but that doesn't work, if she has decided to close the door on you, all you will end up doing is handing her a bit of power that will make her lose some respect for you.