I know what you are going through at this time because I know that many of you make the mistake of saying I luv u to your girlfriend!
Follow these 8 proven tips and I am 100% sure you'll succeed:
1. Ask her for forgiveness and tell her that you'd not do it again.
2. Talk to her like a friend.
3. Joke and make her laugh.
4. Praise her (e.g. saying that you cannot get another friend like her and you would regret to lose her).
5. Go out with her, don't be a miser and buy everything she wants!
6. Sometimes you should look at her affectionately and when she ask you what are you looking at, then just tell her that you are really happy to have her as a friend.
7. When she's in trouble or in sorrow, be the first to console her.
8. Finally, do something that would provoke her to tell you her feelings or you when you are going to drop her home, then let her know how much you love her and want to have her as a life partner! (I'm saying this because you said you love her) :)
I hope I answered your question!