I had a similar problem last summer.
My trainer noticed that when I focus I bring my right leg up, causing my stirrup to slip. Theye could be a little quirk that you have that is causing that.
My trainer had me ride with a crop behind my elbows, to help me straighten out and keep it even.
It just takes time, but you could also try shortening the stirrup one hole and gradually bringing it down?
"Think of riding as a science, but love it as an art" ~George Morris