[Coler]

Grasping these three things really changed my understanding of and approach to the left hand, and I wish I had understood them earlier:

1. To fret a note, you need to apply pressure enough to bring the string into contact with the fret immediately ahead of the fretting finger, not to pin the string to the fret board. You need much less pressure than most people appreciate and playing with the lightest possible pressure applied will help your music to flow as desired.

2. Pressing harder won't help to get the notes clean, if your left hand fingers are muting/buzzing other strings- you need to get your technique right at the finger tip to solve that.

3. Your guitar needs to be set up correctly for the way you are trying to play. Between set up and technique, you should be able to fret notes including in a barre without using your left hand thumb. I'm not saying you should play that way, but give it a go as an exercise - play through some scales, fret a few chords.