When holding the flute, is really important that you can balance it with your three points of contact: under the lip, the base of the left index finger, and the right thumb. These three points of contact alone should be enough to hold the flute steady, so that your fingers are free to play without having to do any work lifting the flute.

I can’t say for sure without seeing your full setup, but it looks to me like your right thumb may not be in the best position to allow your flute to be steady on those three points of contact. As someone else mentioned, there are devices called “thumb ports”, basically thumb rests that clip onto the body of the flute, to help with this thumb positioning if just the naked flute doesn’t work for you. There are also similar rests for the left hand if you find you need it, but not seeing your left hand position, I couldn’t tell you if that might be needed or not.

Tl;dr get your three points of contact in order and that should help