I'm quite new to this too and recognise the 'off' feeling only too well! The way we've handled it is to have some quite firm arrangements about the time leading up to my husband Astraeus's date with his lover Daedalea, and afterwards. He usually sees her every week for dinner on the day he's working in her city. He phones me after he leaves work and while he's walking to her place, which gives me the chance to hear his voice, and for him to reassure me of how much he loves me and is looking forward to being home with me later. He says goodnight to our son too then, and then after he rings off, I go into a sort of 'cocoon' mode where I busy myself with our son, doing all the usual 'evening zoo' stuff (homework, dinner, bath, bedtime).
Once all that's done, there are usually only two hours or so before I know he will be heading home (he catches the train, so it's always at the same time). He always phones me as he's walking to the station so I can hear his voice again, hear him looking forward to seeing me, and we can reconnect. I usually ask if he's had a nice evening with Daedalea, but we don't generally talk in any detail about what they did until he comes home (it's a nearly two hour commute, damnit). When he walks in, we always give each other a huge hug and tell each other how much we love each other, and then we usually sit for a little while on the couch and talk about his date before going to bed. The amount and detail of the conversation depends largely on how I'm feeling - Astraeus is open about everything and I can ask him anything I like, up to what I'm comfortable with hearing about.
As I said, we are quite new to this. Our 'arrangement' has been officially in place since late November - before that it was much more chaotic and therefore painful, so we decided to make an actual written agreement about how/what/when etc., and that has really helped. I am gradually getting better at handling my difficult feelings, and have also been experiencing some compersion brought about by being able to see how much joy Astraeus and Daedalea bring each other. Daedalea also spends a lot of time with us together, so we have had the opportunity to become very close which has also helped.
I guess, in a long-winded way, I'm saying that what worked for me was:
1. having good arrangements and boundaries, which help me to feel safe, valued and loved;
2. taking things a step at a time - trying to not take huge steps that cause too much pain, but also making sure to keep moving forwards.
3. having a relationship with Daedalea that keeps her real to me (as opposed to 'shadowy other woman')
4. taking an active interest in Astraeus and Daedalea's relationship.
So far, so good?