while your commitment to the relationship is highly commendable, your excessive amount of effort it a little frightening. You're moving way too fast for her, and you've just got to breathe a little. You can promise her many things, and give her beautiful gifts, but it's not going to change how much she loves you. That's solely up to her. And i promise you, if she does, she will remain loyal throughout the whole period of 2 years and it will end up fine.
If you're planning on a marriage, you should start practicing a little now. An important aspect to it is PATIENCE. Another is communication. Sit her down a little and ask her if she feels you're rushing things. Ask her why she feels the need to wait, but more importantly, tell her you will wait with her. That's what she needs to hear, no matter what her reason is.
Good luck and congratulations. :) You're ready to hold out, remember?