I heard that we aren’t held accountable for are thoughts But we will be if we keep thinking about it. For example if someone likes a girl and keeps thinking about them is he Sinful or vice versa.
Allah does not hold you accountable for the bad thoughts that cross your mind providing you do not utter, dwell, or act upon them. You are not sinful providing you lower your gaze, do not chitchat, or free mix with her as she is still a non-mahram to you. If you are serious about marriage, approach her father for her hand.