This morning I had a lustful thought of my wife and I became aroused. Then I touched myself inappropriately for a split second. I suddenly stopped. Did I commit a mortal or venial sin?
So long as you didn’t “snap” or the lustful thought wasn’t about some other girl, I wouldn’t call it sinful.
Besides, the fact that you immediately stopped is proof enough that you want to follow God and not arouse yourself with sinful behavior.
No sin was committed thanks to your faith