I'm confused. My new boyfriend left a 15 year relationship, 4 years ago. They still talk to eachother amicably and regularly on a roughly weekly basis, lately more often. She has recently moved in with a new man then left again after a week or so, and is now sleeping on friend's floors until new accommodation is secured. We are both in our early 40s, the Ex 50.
Because my boyfriend left her after an extremely long term relationship 15 years+, obviously she was devastated, as it appears was he, as he does say that he will be racked with guilt his remainder life, and that every unhappiness she experiences henceforth is 'his fault', because if he hadn't left her, no bad things would ever happen to her.
Her recent trauma has meant she has called him more often, for instance whilst I am at his house at night, again in the morning.
I understand she needs his support whilst she goes through this trauma, although she does have other capable friends for support too, but it has resulted in my thinking it best I let my boyfriend go temporarily in order he can visit his ex, help her practically, support her emotionally, etc. because his nature is such a caring one that he cannot deal with the worries of two women at the same time! She is having a trauma ; I am starting to feel jealous of the support he offers her.
Resultant, he has suggested today that because he feels he cannot give me 100% because he is always worried about his Ex, that perhaps he is not ready for a relationship with me.
We have been seeing eachother for a couple of months, he spends most weekends with me. It would scare the daylights out of him if I told him I had very deep feelings for him, so in order to stay with him, I realise I am going to have to hold back my feelings forever 
but I can handle that: I have not too distantly come through a violent relationship - to a degree where I and my children would have been murdered had I stayed with this man, of that I have no doubt -
so I am also slightly wary of commitment.
I am convinced there is absolutely no funny business going on with them both incidentally - not least the fact they live hundreds of miles apart although he has spent weekends and a week's holiday with her, staying in her spare room or at hotels. But not since seeing me. I am not rationally overly concerned they will reconcile as they both insist they don't want to.
What I need to know, is how to conduct myself in this relationship, in order to accommodate his Ex.
Iaccept she needs his support indefinately, I accept he has a close attachment with her indefinately.
So how do I stop my feelings of mild jealousy creeping in? Otherwise, as has been demonstrated, it distresses him to the point where he cannot feel ready to offer 100% because he wants both of us in his life, although I have no doubt he would prefer to lose me rather than her. I want to support him as he supports her I suppose, because I would not dream of asking him to choose between us (I know I would lose!) as that just isn't rational. For instance, she wants to meet me, he relayed, in order she can apologise for seemingly coming between us and causing any upset. I think I would feel quite intimidatedhaving to meet the woman he has spent 15 years with, and me only a few months! Do you think this would be a beneficial situation to any of us though?
Should I just let him go completely? I have two very young children who are already attached to him.
Lastly, before the flamers rush in to fan a fire, he is a decent man of consistently good standing among friends, family (his and mine), colleagues, acquintances, with no prior history of infidelity. He left his Ex because he 'fell out of love'.
Unfortunately, he knows Mumsnet is my hobby, so there's a good chance he may read this :/ so please be considerate in your responses, it is me who requests advice, not him. I come here because firstly I do not really want to divulge such private matters to close family and friends, but also because the unbiased views of strangers can be helpful, as I have certainly been helpfully advised and supported with my past trouble.