I had a quick look over your posts to date. The solution to your problems is ultra simple: get a teacher. Here you are having problems with the left hand, and in your other posts it is the right hand. You haven't mentioned back pain or shoulder pain yet but I guess that will come.
George's suggestion illustrates the problem. You should be able to play barre chord's with little or no force from the thumb. It sounds like you are using the "group of death" approach when you are using you thumb and fingers as a clamp. Rather than your thumb, you use the weight of the right arm to give much of the pressure necessary.
There is another recent thread about barre chords illustrating how you may only need to hold down a few strings not all six. My guitar teacher doesn't even introduce full barre chords into his standard approach for an average student until he/she has been learning for 2-3 years. For much repertoire there is not need. However, in the very first class much of the time is given over to a series of stretching exercises for the fingers. The other major theme was posture and holding the guitar.