All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If this girl reverted to Islam, then this is a great blessing that Allah, The Exalted, bestowed on her. We ask Him to bless her with steadfastness upon the truth and help her adhere to it. We also ask Him to reward you if you were the reason for her reversion to Islam.
There is no doubt that communicating with her – whether before or after her reversion to Islam – is forbidden and a means leading to Fitnah (temptation). Hence, it is incumbent on both of you to repent of that by quitting this sin, regretting having committed it, and resolving never to again commit it in the future. It is obligatory on you to cut off your relationship with her and refrain from chatting with her, even if it is done at far intervals, to avoid opening a door for Satan to lead you to fall into sin with her, especially given that both of you are emotionally attached to each other. You should rather adhere to patience until Allah, The Exalted, facilitates your affairs to afford marrying her. Allah, The Exalted, Says (what means): {And whoever fears Allah – He will make for him of his matter ease.} [Quran 65:4]
Your mere promise of marriage to her is not considered a valid engagement. Under the Sharee‘ah, an engagement is a promise of marriage between two parties; one of them presents a marriage proposal and the other accepts it. They agree on that, and their agreement entails deeming it prohibited for any other man to propose to this woman until the suitor gives him permission to do so or he cancels the engagement.
Allah Knows best.