I think you need to do something now.
Hi. My question would be this: Did your MIL cheat on your FIL? Very often a spouse who comes from a family where the mother and father have experienced infidelity will go into adulthood believing whomever they marry will do the same thing. I'd be curious to know whether that happened to your husband's parents.
I also think it would be a very good idea for the two of you to see a marriage counselor. Your husband may not even realize how his mistrust is affecting your relationship with him. He needs to see that you feel as rejected as he
He is insecure because he assumes you are capable of cheating, since he himself is capable of cheating.