What is the ruling on one who invoked upon herself not entering Paradise if she were to marry someone other than the person she loved? Should she marry the suitor who has now proposed to her, and is there any expiation for that?

A Muslim is not permitted to supplicate against themselves, lest they perish due to their supplication being answered. Repentance from such an act is obligatory. The supplication made against oneself does not constitute a religious impediment to approving the marriage to the young man if he is pleasing in his religion and character. Seeking guidance (Istikhara) is recommended.