In measure 20, when a double stop is played with the first finger played on both the A and D strings. My fingertip isnt thick enough to lay down both strings like john shows in his video. If I turn my hand and lay my finger long ways across both strings it works but it puts my hand in an awkward position that is hard to move out of and feels unnatural.
I believe the way to do that is to press in such a way as to bend the first joint (of your finger) backwards enough to cover both strings. This is called bridging and is a useful skill. If you could post pictures of what does and doesn’t work, we could better help find a working solution
Sometimes having fat fingers can give an advantage! :) Your finger is probably wide enough, but it does take a little work to master playing two strings with one finger. You may have to turn your hand slightly!