It sounds to me like you have some sort of sense of ownership or possessiveness with M that you don't have with the other women in your life. Perhaps because you view her as primary, on some level you feel that there is some kind of "right" you have that others shouldn't. This could be a very old, familiar pattern of relating that you might be carrying around with you. If you think about it, does the dynamic of your relationship feel similar to any others in your past, even from childhood, where you felt allegiance to someone and then they didn't seem as faithful to you as you were to them? Because there is some association you are making here, that her having other lovers is a threat to you somehow.
Anxiety is usually accompanied by a heaviness in the chest, and irrational fears. There's a book called Mental Health through Will Training by Dr. Abraham Low (he started Recovery, Inc.). He wrote about how to deal with anxiety and irrational fears by doing the following: change our insecure thoughts to secure thoughts, move our muscles to overcome the defeatist babble of the brain, do not attach danger to the symptoms, bear discomfort in order to gain comfort, and endorse yourself for your efforts no matter the outcome. In other words, when you're feeling this way, go out, take a walk, do something physical, and focus on thoughts that make you feel secure. Know you will get through it, and acknowledge yourself for facing it, even if you feel like you're flailing, or failing.
One thing that helps me when I get jealous or possessive is that I remember that loving someone means I want them to be free and happy, and that they deserve all the love they find, not just mine. Reminding myself of that is tremendously helpful.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein