I meant to reply earlier, but I could not verify if my thought were legit. I was thinking about people usually being right-handed or left. I am pretty sure we have a dominant chewing side to our mouths. My idea concerns working some facial muscles more than others in chewing activities or clinching. Some people sometimes hold objects on one side of the lips, such as a pen or a straw. I am just brainstorming why one side of your facial muscles feels weaker.

I am going to recommend an exercise that is in the Andante level. But first, be aware of these cautions.
Aperture Corners
It is very easy to get confused when searching for the best way to improve, given the environment of conflicting views in which we live. For this reason alone, it is beneficial to treat technical development as experimentation. What works for you may not work for someone else.

Danger lies in locking the cheek muscles or clenching at the sides of the mouth. This is a common practice that can severely diminish efficiency and hinder flexibility.

Allow the tension at the Aperture Corners to determine how much work needs to be done in the facial muscles. The louder you play, the more work the facial muscles will need to work to hold the Aperture Corners in position.

Keep The Jaw Down
Be sure to keep the teeth apart. Lowering the jaw allows the Aperture Corners to spring into action and eliminate clamping at the center of the lips.

Learn to recognize the inward, horizontal movement of the Aperture Corners. Add to this an understanding of the role of the tongue and your upper register will improve dramatically.

Place the index and middle finger extended like a closed peace sign. Put the two fingers, into the mouth with the fingers pointing into the mouth. The index finger side is on the bottom teeth and the middle finger is under the top teeth. Say aah-ooh and bring the aperture corners toward the fingers. Repeat. Exercise 2: Put the little finger in the middle of the lips and grip it with your aperture corners, not the top and bottom of the lips. Let the lips naturally roll out, do not smile.

Hope this helps.
Ronald Carson