Welcome to the wonderful world of sex!
You're both 19 years old, and thus old onough to make your own decisions. But if you have an agreement with her mom, that you should talk to her first, I think you should do just that. Have a meeting with her, under relaxed circumstances of course, and tell her that you're both grown very attached to each other and that you love each other very much. That should ease things in a little. Then tell her that you've discussed the matter between the two of you, and that you'd like to begin using birth control. I don't see why she shouldn't understand that, it's only a good thing that you worry about birth control. I say go for it, and be sure to have lots of fun!