SO... what kind of before/after care
do you get from your husband? Before he has sex with K?
And what kind of before/after care do you get from K? Who lives in the home with you?
Because I'm not hearing you get any sort of before or after care and that probably adds to the jealous UGH feeling of "What am I? Chopped Liver?!"
You need support and nurture and while you sound willing to share your husband as the hinge, and they are willing to hear your upset and forgive you... if they cannot mind reader how to before/after care you... you kinda have to know your OWN self and announce what needs doing.
I don't know if that helps any.
But there is my impression.
Also make sure you have done the obvious health check ups -- with nursing babe cosleeping? You are still awash in hormones from either recent pregnancy and/or nursing hormones. That jacks up the hormone soup your brain floats in, and it can color your feelings/perceptions/thinking. I nursed our kid for many years and while it was one of the best things I did, I remember the first 2 years I felt all up and down heebie jeebie! If you are taking the mini pill (for birth control while nursing) check it's chemistry works ok with your own brand of body chemistry or any other meds you might be on for other conditions.
How is your sleep with a baby around? Sleep disturbances (body bucket) can also cause wacky in emotion (heart bucket) and thinking/problem solving (mind bucket) and they all tie to your spirit feeling flat. (Soul bucket.)
Some BCP's can mess with your emotional meter too. Depending on their formula -- I just cannot deal in a triphasic. I need a monophasic so it is EVEN all the way across not more bopping up and down and all around.