If you don't want to lie.. then don't lie.
If you discover you have made a promise you cannot keep then you can always say so.
What's more important pride "I would rather break my promise in secret than let her know that I made a promise I couldn't keep" or integrity "I would rather be honest than lie".
If you are unsure whether you can keep the promise or not that too can be stated honestly. If you tell her you aren't sure if you can forever keep the promise, that you may have been hasty optimistic scared or feeling pressured when you made it then you at least are being honest and forwarning her if you find that to be true.
I suggest that you might suggest counselling.