OP feels aggrieved that the future she wanted is not the future she appears to be walking toward, and I'm paraphrasing one particular objection to her DS's partner as highlighted in her post above. But it isn't her future that's under consideration here. It's that of her DS. And as you also seem to be suggesting, the question of whether GF makes DS happy is one for him alone.
OP is disappointed in her son's partner and wants, I suppose, some validation of that feeling. As always, some posters have done just that, some have offered ways to manage the situation, and others have told her she's being outright unreasonable. Such is the nature of a discussion board.
Anyone is entitled to feel what they feel; it won't make one jot of difference whether others validate this or otherwise. The problem is that neither option is productive or likely to achieve anything positive whatsoever. It certainly won't provide any viable solutions to this particular dilemma.
'I will just have to try harder' also isn't an effective solution. Why should OP bust a gut trying to make herself liked? IME, people usually sense this and it tends to incline them less, not more, to your favour. Plus, given OP's openly admitted hostility toward the woman, the mask is bound to slip sooner or later. Waiting in hope for them to split up is equally unproductive. If my in-laws nurtured a similar hope on my account - and it's not inconceivable that they have - they will have been sorely disappointed. 25 years later, here I still am 😀
There are many possible suggestions upthread as to how this situation might be approached more positively, and part of that might unfortunately involve managing expectations of the relationship. It's very likely this relationship will never be what either side wanted, but given time (and hopefully less chatter amongst the family about how impossible GF is - even if she is) it might be possible to arrive at some form of workable relationship.
If it's a question of making her feel welcome, unfortunately it seems that ship has already sailed. She clearly feels uncomfortable but isn't distancing herself entirely and is continuing to visit. Best to take that as a positive and build on it. Four years isn't all that long, and a savvy parent would opt to play the long game here.