Hello, Alwaysrejoice!

I do not know how you can stop feeling your feelings. You may be able to create an environment in which those feelings have less force by

• obtaining reliable information on your husband's intentions, and
• gaining insight into your worry patterns and tendency to worry.

Depending on the severity of the anxiety, I can also imagine a professional possibly prescribing anti-anxiety medication for use on a regular or as-needed basis. (Does one of your current meds have a primary or secondary anti-anxiety use?)
Is it irrational? Has your husband (or mother-in-law) given you any reason he's thinking about divorce? No need to answer; these are things to consider.