All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Not fulfilling a promise which is not accompanied by an oath is considered a bad moral conduct against which the Law Maker [i.e. Allaah] warned us, even if it is made to the people, let alone if it is made to Allaah and it is for avoiding disobeying Him. Therefore, fulfilling such a promise is even more confirmed and breaking it is even more heinous.
Indeed, Allaah praised the believers for being truthful in their covenant with Allaah. Therefore, you are obliged to expiate for breaking your oath because giving a promise to Allaah is considered an oath according to the Hanafi School of jurisprudence as stated by the author of the Hanafi book "Al-Ikhtiyaar". Also, giving the value in expiating for an oath is permissible according to the Hanafi School as well, since you requested to answer your question according to this School.
For more benefit, please refer to Fataawa 102811 and 82841.
Finally, you are obliged to stop talking to that girl, and regret and repent for what happened between you and her because it is forbidden in Islam to establish an affectionate relationship between the sexes outside marriage as we clarified in Fatwa 81356.
Allaah Knows best.