You are responsible for looking out for your own mental health, emotional health, physical health and spiritual health. ALL your healths.
You sound troubled with it -- what kind of feedback do you need from the forum to best help you?
I am sure I am capable of loving more than one person, and of sharing a person I love, but I am incapable of casualness
You seem to know this about yourself.
To me? It sounds like you want to date and eventually end up in some kind of committed maybe polyfidelitous open relationship model?
Is that it?
It sounds like you broke monofidelitous agreements with your husband by starting up with a woman who seems to enjoy a polyamorous style that is casual. Why do you write "affair" like that? It is or isn't. Since he's not sounding thrilled, it sounds like you did NOT have an open marriage agreement in place. Where is your head at with the marriage? Still wanting to keep it? Over it? Where is your willingness there? Where is his?
Are you planning to ask for more commitment from her? Ask to break up with her? Where's your head at there with the GF situation? Where your willingness on that?
If monogamy is not for you, could you finish with the divorce process cleanly first? Is open relationship totally not possible with the husband? He's still your lover while he's dating some new lady. So... is the hurt for him in the betrayal? Has this hurt been healed or can it be healed so you can be together? If you both want to be apart -- why are you sharing sex still? If you want to be together -- why are each of you pursuing extra relationship while THIS one is broken and in need of TLC? Get clear on what you both want out of your relationship and then both of you behave like you want that. This mixed up business just leads to more mixed up.
Divorce is a pretty major hit on the stress scale.
It doesn't have "dating new person" on there but you could tick it as having a new "family" person in your intimate circle with the GF. You got other stressy things going on? Get a sense of your stress score there -- are you at risk for getting ill?
Again -- YOU are responsible for your long term health and well being. Taking all this stuff on at once... could you REDUCE the stress load anywhere?
I know it could be hard to feel or seem hard to do. But choosing to straighten up some things by making some firm decisions could be worth it to gain better long term
healths and well being faster
Get your own wants, needs, and limits clear with yourself. Then check in with the husband and the GF and get the clear wants, needs, and limits there. Plan your life strategy along cleaner lines.
Right now this reads like a mess! I am sorry you are hurting.