Yeah, I know what you mean. When I first started playing (not all that long ago), bar chords about killed my left hand and forearm. I was doing the same thing, pressing like h*ll with my thumb, cranking on it for all it's worth. But now, not so much. I've sort of developed a technique for myself where I hold my thumb a little lower on the neck so I have a better angle on the bar.

Of course, it's still tough on an acoustic guitar.

Like anything, it'll get easier with time. :)