@Jonti23
You say that she is exhibiting mental health issues by harping on about this, and she should put it behind her and get on with life.
I see what you say and raise you this - "hiding things from yourself deliberately to be fakely happy is a sure-fire way to poor mental health."
The man has shown her that he has low standards and thinks he can buy women. He has been sexually intimate with other women when in a monogamous relationship.
She ought not to brush that under the carpet, for it never really goes.
The trouble is not the thinking about it, it is the NOT thinking about it.
In today's world, many men are of poor standards, borne out of patriarchy where they think they are the bosses and have all the power and can do as they please, and just make the silly woman shut up. But this is no longer the case and we can and do leave men who are not good enough.
How long this will go on before men catch up and fix up, who knows. How many women will leave and just be single, who knows.
I feel for the OP here because for no good reason, her partner has just up ended their relationship and shown her that she is not special to him and he will let other women get sexual with him, where she has loved him and honoured him. He has not honoured her or their relationship, and has shown that he will give money for sexual activity with sex workers. And she is gutted about what this says about him and how much she means to him, and the sort of man he is.
Stop asking her to lie to herself and suggesting that this is the way to be happy.