I am utterly new to guitar so it's likely I'm just not "getting" something, but anyway - I have some difficulty fretting - specifically changing chords - while standing.

If the guitar is a lever in this scenario, my belly is the fulcrum. It feels as if my strumming/picking arm (my right) presses the body of the guitar back towards me, and levers the neck away from me, while I play.

Hence when I change chords, the neck moves as soon as I release the pressure with my fingers to I move into the next chord. I end up often messing up the change, or going through some seemingly unnecessary contortions with my fretting hand. For example, going from simple Am to simple E I tend to keep my middle finger on the G string until I have my index finger in place. Makes the changes less smooth than it seems like they should be.

I'm not sure what I should do differently with either arm. Hook my thumb over the top of the neck? Lenthen or shorten the strap? Lose some weight?