Men are what their mothers made them.
~Ralph Waldo Emerson
This is a very important letter and I am glad you have taken the time to write us. I can only imagine how difficult your situation is.
I think you will need to go toe-to-toe with your husband on this one; otherwise, the relationship is headed for rougher waters.
It is the consistency of his behavior and the chronic feeling that you are not valued that must be addressed. Occasional separate visits are fine, of course, but it is the ongoing feeling that you are being dismissed and not valued where the problem lies.
Here is what I suggest:
Preempt his plans by explaining that you would prefer to go along, or that he at least ask you about going, since you are assumed to be available to be with the children. Be clear that him making plans that require your participation without consulting with you isn’t okay.
Suggest that you both pay for you to go along and graciously accept his mother’s contribution.
When he says he will not see his mother anymore, confront him with the challenge to think of something that falls between capitulating to her and an all-or-nothing demand on your part.
If he makes plans without consulting you, ask him to arrange for someone else to watch the children for that period of time, as you are not willing to do this under the circumstances. What you do during that time is your business. It may include staying home, going away on your own or with friends, staying over at a friend’s house, or catching up on your reading. The point is not to take the role that is forced on you, and to make him realize he must take greater responsibility for his decisions.
When he says it isn’t his mother but his weakness, ask him how he plans to become stronger. Offer your willingness to go with him to a couples counselor to help with the process. You may locate one at the link above under “find help” or here.
Your feelings are very understandable and I would not be shy about your concerns and needs. It is time for your husband to grow up and for you to grow stronger.
Use “I” statements about the situation. This means you say things like: “I don’t feel valued by you when you choose spending alone time with your mother.” Using “I” statements keeps the encounter centered on expressing your needs and feelings, not on blaming him.
Thank you for writing to us.
Wishing you patience and peace,