No particular advice for you sunflower - just to say I am in a very similar position.
I split with my husband (for the 2nd time) over 4 years ago now. We have not divorced. We also have a very amicable relationship, have 4 kids between us - my 2 and his 2. I doubt either of us will want to get married again. I am in a new relationship - not too serious but has lasted over 2 years. I'm happy with this because we live apart and I still feel I have my freedom & independence.
My ex lives alone and also loves it! He's not interested in any relationship other than one-night stands from what I can make out!
I don't want full closure for the sake of our children (even though they are now adults of 24, 25, 26 & 27). By divorcing their father I somehow feel (maybe stupidly) that I am divorcing them too - by putting a final legal end to our married relationship. (I don't think the kids themselves are bothered either way though - it's us!!).
It doesn't seem to bother my present BF I am not divorced - I think he knows I am not likely to want marriage to him & probably likes it that way anyway. He's my age (47) but has never been married & doubt he wants to now.
I must admit, I do think in the future it is more likely to be I who instigates divorce, and I must admit if he did, there would be a part of me that would feel sad - even though I know we will always be 'friends' at the very least.
So _ i fully understand your dilemma. All I would say is - do whatever feels right. There is no right or wrong thing to do. You may well find either your new partner or his might put the pressure on one day for you to divorce - cross that bridge when it comes to it.