I had pretty much the same problems you do with my 10 year old. Honestly, I just got tired of it, ditched the 'lunge/groundwork/start-from-the-beginning' method, got on and rode him like the devil for a good hour or so. I cantered, galloped, chased him up down and under everything. No begging, no pleading, just flat out ride-'em-till-they-lick. Keep it changing constantly, get your adrenaline up, scream, holler, and yell. Be the alpha!
Refused, herd sour, barn sour, reared, bucked, cowhopped, went every which way, ignored commands, etc.
Listened. Obeyed leads. Eager and willing to go out and let out those long legs. Happy. Riding finally enjoyable again.
Sometimes, you gotta ditch the plead-and-beg. Sometimes you just gotta take your boot and stick it right in his behind.
Wait! I'll fix it....