Summarize:

While it is painful to accept, your girlfriend may have moved on since your break-up. Before approaching her again, keep in mind that she may not want to get back together with you. If she rejects you, don't take it personally. Maybe there was a reason why you broke up in the first place. Calling her might feel too forward, so ease in with a text. Let her know you are thinking about her. Maybe you heard a song that reminds you of her? Or, you re-watched movie that you two had seen together. Keep it light and friendly, and ask her how she's doing. You might say something like, "Hey! I was walking in the mall and I heard our favorite song... it made me think of you. How are things?" If she doesn't text back right away, don't despair. She might just be thinking of the right thing to say. If she responds positively, consider asking her out to a social event as friends. You could invite her to a school basketball game, or to eat lunch together in the cafeteria. See her in a public setting with other friends so she feels more comfortable. Make a list of your girlfriend's favorite things and activities. Does she have a favorite musician that is playing an upcoming concert? You could find tickets and surprise her with them. Or, if she just got her driver's license, you could find a cool key chain that is relevant to one of her hobbies (like a tennis ball key chain if she plays tennis). However, you don't have to spend money to show that you care. If she told you she has a test coming up, call or text her afterwards and ask how it went. Offer support if she's feeling anxious. Simply being thoughtful and listening will show that you want to be involved in her life.
Do not expect that she still likes you. Send her a text. Ask her to spend time together as friends. Show her that you care about her.