I was ditched by my current man a few months in, because his ex was having a crisis.
She rang him around midnight and must have known I'd be at his place. I'm afraid I couldn't deal with that, with him walking off into the bedroom to talk to her on the phone.
It's now clear a year and half later he was still emotionally attached to her, and we did make up again after a week or so apart.
But I have never bought this 'just good friends' concept because I have direct experience of that.
I was friendly with 2 of my 3 significant exes to the point I'd be chatting to them on the phone for 3 hours in the evening, yet I knew back then they still held a torch for me and this contact whilst supportive in that it helped them deal with the break up, also meant that inevitably they couldn't completely move on if I was still always in the background.
Last year my man's ex's friend asked him to visit them over Christmas at his ex's house and to my dismay he did actually dither over the decision. That hurt, but I pass it off now as perhaps him not having significant enough feelings for me then that he preferred to spend a holiday with his ex and their mutual friend rather than his new girlfriend. Or that he wanted to please everyone equally, as is his nature, but couldn't find a way of doing that in this instance.
A Christmas card, or once in a blue moon email or text is fine. But if you no longer live in the same part of the country and so don't socialise anymore, well, an ex is part of your history, your past life, and in my opinion they belong only in your past.
It's an attitude I have only recently learnt to apply to myself, too.