Confused about the future

I (26M) is in an interfaith relationship for the last 4 years. Everything was going fine and suddenly my GF (25F) is worried if she would regret her choice in the future because if she marries me she will be practically out of her religion (pentecost christian)
I am a Muslim and we have decided that neither of us are going to convert for the sake of marriage. She is from an orthodox background and my family is not that orthodox.

We were in a live in relationship for 1 year and recently moved back home to prepare for moving abroad. Because of this uncertainty I gave her an ultimatum that only if she stays strong in this relationship I will continue or else its better to end it up here to save from future heartbreak. But she is like no matter what she wont leave me

I am really confused because I really love her and had lots of dreams with her in the future but also I dont want to waste my efforts in an uncertain relationship.

Does anybody here know of inter religious marriages involving muslim groom and pentecostal bride?