I think lots of people who are seeing more than one person at a time have guilt now and again, maybe especially if things are going well.
I personally decided Jesus was AOK with me being super happy so there was nothing to feel badly about "morally" since my husband said he was fine. Learn to trust your husband. There is always a chance that he is a big old liar, but until that turns out to be the case, assume everybody is happy if they don't say otherwise, and enjoy how much love you have in life.
Hmm I guess since you cant seem to shake off the guilt this time...is there a new
dynamic you want to, or have, explored with your partner like BDSM or some specific kink that you haven't brought up with your husband...or are you fantasizing about having both partners live with you or having kids with them when you think DH would say HELL NO. If it is something like that, identifying it and/or discussing it with him may help.