Did it occur to you that your friend might have had a different point of view from yours? Yes it did, hers was that I was no longer useful to her so she stopped contacting me.
Oh right. You asked her and that's what she told you?
I mean, your assumption may well be right. She might be a total user who was using you all along. There are such people. Is it possible though, that she might have viewed it in less black and white terms? Or, possibly, had things going on in her own life at the time, that you knew nothing about, that could have influenced her behaviour? Is it possible that she needed compassion rather than judgement?
The older I get, the more I realise that the way people behave towards me is driven by what is going on in their own lives, not by anything to do with me. I'm just collateral damage (and vice versa, sometimes) when things are going badly. Given time, space and the odd kind word, most friendships can be kept going, for their own full, natural life.
Thing is, people drifting away because a friendship has run its course is normal and natural. You can choose to describe that as people dropping each other because they're no longer useful to each other, if you like. That's a very transactional, cold-blooded point of view.
Isn't that what you did to her though? You told us it was you who cut her off, not the other way around. She let you down, you decided she no longer met your strict criteria for the honour of being your friend, so you dumped her. Because, as a defective and imperfect friend, (whose own explanation for her behaviour was of no interest to you), she was no longer of any use to you.
There are two sides to every story. Maybe if you told us more about what happened, it would emerge that she was an utter selfish user and you were well rid. I'm not saying my 'alternative' perspective is correct. I'm just trying to prompt you to think in less black and white terms.
I have / had a friend who I'm pretty sure has BPD/EUPD (is that what we're talking about here, rather than bipolar?), who had an extremely rigid, self-centred outlook on the world and other people, that didn't really allow for the idea that people had other stuff going on and were not solely motivated by their feelings towards her, even by calculated thoughts about her, in all their interactions (or failures to interact) with her. She expressed a weird combination of assuming everyone was thinking about her a lot more than they actually were, while wishing them not to think about her at all, yet actually craving attention, on her own terms. (I think this 'everyone's looking at me' effect is common with depression too). Also, she couldn't seem to tell the difference between feelings and facts. if she felt something, that made it a fact and no real world evidence was relevant or necessary.
I've no idea how you are, or if there are any similarities. Just sharing as food for thought, if helpful.
As for your partner, the obvious advice is to talk to him. Explain how hos behaviour makes you feel. Ask him, in a non-accusatory way, why he goes about things the way he does. Be prepared to listen and to try to understand. Maybe he is being disloyal in the ways that matter in a relationship. That is quite possible. Maybe if he understood that he'd be motivated to stop. Maybe not.