Am I obligated to marry a girl I am engaged to, despite my doubts about her commitment and actions, and my fear of her betrayal in the future?

Being alone with a non-mahram woman and looking at her are impermissible. There is no difference between you and the other man who caused doubt in your heart, as both of you are non-mahrams to her.

If the girl adheres to the law of Allah and repents to Allah for what she has done, and her going out with you was due to ignorance or poor upbringing, then there is no blame upon you in marrying her. However, if you see that she is otherwise, then leave her.

Our advice is for you to stop meeting this woman, going out with her, and speaking to her until the marriage contract is concluded, even if her family permits it, for your religion forbids you from doing so.