Dear Nina, 

The greatest thing we can do for God is not going away to be a nun, but doing his will.  

I am sure that when you made your promise to God it pleased him, because at the time you were nearly certain that was his will. But if he, after you made the promise, shows you that he wants something else of you, then do what he says. Don't worry, it is never a sin to do what God wants even if you promised him something else.  

Now, there is still something very important for you to work out. From what you say above it is your feelings that are swinging back and forth. It would be unusual if this did not happen, our feelings are like that, fickle as the wind. So, if you base a decision or commitment on feelings you are going to be very confused in the future, because no matter which you decide for (marriage or nun) it will only take a few days for your feelings to swing back again to the other side.  

You have to seek in your soul, in the way God has been leading you, in the principles of faith, in the things that matter most to you, in the advice of someone you trust, what God really wants of you, knowing that no matter what it is, your feelings are going to be sometimes for it and sometimes against.  

Then, knowing that, go ahead and opt for what he wants.