If you don't want to lose her you need to give her the time she asked for if you continue to contact her while she is making up her mind what she wants she may decide that you aren't the person for her because you didn't respect her wishes. She said she now needs time to think and if you are continually contacting her it will only confused her more. give her a week or two without contact and then you can call her but don't mention anything about the relationship because she may not be ready to give you an answer yet. If you love her give her time even though it may be killing you she will respect you alot more if she knows that if she needs time you love her enough to give her that and you should also tell her that, says to her "Even though this is killing me right now, I love you enough to give you the time that you need" this way she know how you feel about it and you are still willing to give her that time.
What you are going to have to do in the meantime is occupy your spare time so you aren't harping on her and the situation, hang out with friends, volunteer your free time, work more hours at work, take a vacation get away from the things that may remind you of her for a little while and just enjoy yourself, do something you wanted to do but didn't because you were with her and allow yourself time to think about what in the relationship bought her to this point in her life that she is having second thoughts and then think of what you can do to change that aspect of your relationship but it's still very important for you to respect that she wants time to herself for now and then you can talk about the relationship when she is ready to do so.