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When you hear your ex girlfriend tell you that she just needs to have some time to think about things, honestly, that can mean a number of things. It may be exactly what she says, that she just needs some time and if that is the case, then you really do need to give her that time to think things over. Not allowing her to have that time is going to guarantee you that she ends up feeling like you are not the one and that she will just decide that breaking up with you was the right thing for her to do and I doubt that you want that to be the case.
On the other hand, sometimes a woman will say that to her ex boyfriend just to avoid having to tell him the truth, that no matter what - she is not going to be coming back to them. You have to be ready to deal with the fact that when she says that she just needs some time to think, she really is just trying to avoid hurting you. You should be grateful for that, because it at least means that she cares about you and respects you enough to not want to hurt your feelings.
How do you know what she really means?
You can take a look at what she is doing right now. If she is dating someone else, or she is moving on with her life and you hear her say that she just needs to have some time to think, it would be best for you to assume that for right now - you don't have a chance with her. Instead of wasting lots of time and energy on trying to change her mind or trying to win back your ex girlfriend, you can spend that time and energy on moving on with your life and finding someone else to date.
Of course, if she still is hanging around you and flirting with you, then she may be really close to deciding to get back together with you. If she is leaning towards reconciling with you, then you need to know what to do to make sure that she doesn't lean the other way and change her mind. For that, you are going to have to try and make your ex girlfriend feel attracted to you again and that should be easy to do, considering that she was at one time your girlfriend so you definitely know that you were once able to make her feel attracted to you.
Overall, you just want to make sure that you don't react to this kind of thing by doing anything that is going to make your ex girlfriend hate you. If you do that, then you can bet that when she is done thinking about you, she is going to decide that she is better off without you in her life.