One of the reasons that everyone encourages new members to post in the Introduce Yourself section is so we can get a feel for what's going on behind your questions... You mentioned in your Intro that you were
taking lessons, but that the teacher wasn't specifically a "classical" teacher...
If your wrist
hurts when you are trying to barre, (and the guitar is still slipping) it sounds to me as if your sitting/holding position probably isn't right yet... and that's a much a bigger question to ask. If you are having pain, something isn't right... and you really shouldn't ignore it...
This whole problem of finding a good sitting position, how you hold the guitar, whether you use a foot stool or a guitar support, the angle your hands contact the guitar, etc. are just too important... and so difficult, even when you DO have the best of teachers.
As you are discovering... you can start to have some very real aches and pains if you're not doing it just right. We get so focused on trying to read the music, find the right strings and frets, make decent sounds... you get tense, you get out of position and after a few minutes, you're a bundle of nerves and tight muscles... and the frustration just starts to spiral out of control. We've ALL been there!! If the guitar is still slipping, you're wasting 99% of your mental and physical energy trying to hit a moving target... and aside from pain and tension, your finger aren't getting reliable feedback because the frets and strings are never in the same place twice... try anything
... (lots of people use non-skid shelf liner on their thigh or in their lap, or a cushion, or a guitar support, or even a strap... just so they can sit comfortably (probably with the neck fairly high) so you can relax and PLAY)
Most of the suggestions that people make for things like barring with less thumb pressure, etc. are meant to be "hints" to help you feel the differences rather than literal "rules"... The main thing is to keep experimenting to find small
adjustments that get you closer to the ideal totally RELAXED secure position, where the guitar isn't wiggling around, you aren't twisting or straining your back, your wrist or your shoulders, and so your hands are free to play.
If you have any doubts, here's the name of someone who might be able to recommend a specifically
classical guitar teacher in your area... guitar foundation.org/drupal/node/554 A classical guitar player (or more advanced classical guitar student) would probably be willing to give you even one or two lessons, just to help you make sure that you're sitting correctly and going in the right direction.
I really can hear and feel your frustration... and I've been there! I looked for ages to find a real classical teacher and had to wait for months to get a first lesson (and had to BEG him to fit me into his schedule) One hour with a really good classical teacher (and the guitar support he lent me during the first lesson) helped eliminate a huge amount of my frustration. I still haven't found an available full time teacher, but I know I would have quit by now if I hadn't managed to have those few early lessons with a really good classical teacher who sorted out how I was sitting and using my hands.