Pleae give some more detain on the eight corner drill?
Just having a partner punch randomly at you from all angles, low/high hook punches, jab, cross, uppercut, stomach shot, back fist.
The 8 corners being your use of dai sau, tan sau, pak sau, chit sau (or gan sau)/jum sau, etc.
You drill this with contant spontaneous attack upon you by your partner. Like what you said your Sifu does to you with your back against a wall and doesn't allow you to bridge. Punching randomly at you from a distance.