It sounds to me like he has just taught you a solid lesson in the fact that you were holding him into a fake frame previously.
Keeping the hind legs engaged is not about running the horse on fast, which happens all too commonly with new riders thinking the hind legs are 'engaged'. Active hind legs happen without flying around the arena at a million miles an hour. So first thing, make sure this is not what you are doing.
Second, have you started any work towards laterals, namely leg yield and turn on the forehand? If not, I would start it. These movements teach the horse to move from the inside leg, and fill up the rein. Which then leads to lateral movement beginning with shoulder fore and working from there.
This work will assist in developing a contact and a soft back which in turn is where your so called 'head set' will come about.
Regarding contact, it needs to be consistent. You can't squeeze and then give it away. By doing that you are essentially dropping the horse on it's head.
Ride with an active seat - your seat should be doing 95% of the work, the leg and hand is there just to support. You seat holds the horse together, it dictates speed, direction, length of stride, tempo, rhythm, gait, stop, go, turn.... If you rely on reins and legs, the horse will be unbalanced, unsupported and ultimately ends up on the forehand or in a forced, stiff frame.