Is what happened between me and the young man, involving mutual kissing all over the body without penetration, considered zina (illicit sexual intercourse)? And must I stay with him despite my dislike for him and my mother's opposition, due to my feeling of guilt about what happened and my sense that I will destroy his life if I leave him?

You must repent to Allah for transgressing His boundaries. Part of completing your repentance is to completely sever ties with this man. This act is not considered zina (fornication/adultery) that necessitates the legal hadd punishment, as zina is defined as the insertion of the entire glans or part of it into an unlawful vagina. However, the repulsiveness of the act is established, and it may metaphorically be called the zina of the eye, tongue, and soul because it leads to actual zina. As for marrying him, we advise you to turn away from him since you dislike him and your mother rejects him.