So I've been practising on improving my technique and overall approach on how to play the guitar. I have readjusted the way i sit when i play to classical style and ive been trying my darndest to keep that thumb in the center of the neck and it works for me because keeping the thumb there really extends your fretboard stretch and it just makes everything feel easier. But after a while my thumb just gives out on me. It hurts so friggin much and i dont know if im doing it wrong or if i just have to get used to it. Am i supposed to keep it extended out (think like hitchhiker thumb) or in the center but kind of curled at the end?
When I get the urge to play classical style, I sit down until it passes.
Your not doing it wrong, your probably doing it too much. Try and combine both the 'thumb behind neck' techique and the classic 'closed whole around neck' technique. If you only use one, you will be limited and may procure some nasty pains like you described. Combine the practise of both, and if it begins to hurt or ache, stop for a while or switch to something else. If your pain is a shooting sharp pain and not a general aching muscle pain then there is something wrong and I can't help you there. Accept I could say just take a break.