For me it was my right cross. Being right-handed my right was obviously more powerful and unfortunately my undoing. My left is actually more 'loose', but my form was always perfect on the hook. But I imagine it was only a matter of time before it went too. But, the guys who hurt themselves in my club all hurt their left...so maybe I was just lucky.
Here's my 2cent: You certainly could go back to training. Since you've had at least one match, you know that training for competition is brutal. Training for a match and general training for health are totally opposite. So, dont train as if you are preping for a match. Just take it easy, focus on form at all times. Perhaps avoid the left hook and focus on combos from an in-tight position. Don't go to exhaustion! If you want, spare no more than 1 every 2 weeks. Sparring for a match gives you the tendancy to over compensate and u may have a tendancy to go beyond your limits. eg. being caught in a bad position and trying to over compensate.
See if u have loose joints and that 'hunch' over look. If u have that hunch, you should train your rear delts, back and traps like a fiend. This will eventually correct that. If u have loose joints....I dont know. I found that I couldn't work around it as far as boxing goes.
Thats fairly general...u should find a good trainer. They along with your knowledge of your limits, can make still make you pretty damn good.
Why don't u look into being a sparring partner? Pros and semi-pros are always looking for a good defensive sparring partner.