Am I obligated to marry a girl whom I promised to marry, and she rejected many suitors for my sake, then I left her and caused her psychological state to deteriorate?

If the girl is religious and of good character, then you must fulfill the promise and marry her. If she is not, then you are not obligated to marry her and you do not sin by leaving her. It does not matter that you promised to marry her, because fulfilling a promise is recommended, not obligatory, according to the majority of jurists. If she is not suitable for you, then inform her of that.