Quoting from a PM I received. Hits hard but it's 100% right as far as I'm concerned. Kind of a shame that the diving season is almost wrapped up around here. Be nice to get out some more, but I guess pool times will be starting up soon.
You have not mastered one of the most basic and important skills you were taught: How to disconnect the inflator hose.
You learned that in a real emergency you don't know how to ditch weights
You learned that in a real emergency you are prone to panic - even after you are safely on the surface.
You learned that in a real emergency you might choose to swim down, rather than dump air from the BC when you are too light
You learned that a DM can't always effectively watch over you and help you - so you need to be more self sufficient.
You learned that your problem solving skills underwater are weak - if you thought you had a bad leak from the tank/regulator - then you should have been thinking about going to the surface- 20 feet away - not fighting to exhaustion to remain at depth.
You learned that signaling a problem underwater to a buddy/DM may not be as simple as is seems.
You (hopefully) will now learn that a stuck inflator is one of the most common scuba gear failures
You learned that over exerting yourself and trying to use brute force to salvage a situation is not always best
You learned that something like dual calf cramps can incapacitate you and prevent you from swimming a short distance even in a calm lake with no current.
You learned that underwater, things can go from fine to a big problem in an instant and with no real warning.
This little incident should teach you a ton of stuff, and all of it should help you become a better diver and safer. You need to rationally critique all your thought processes and actions, work on developing a game plan to handle similar emergencies more effectively and you should figure out a way to survive failure of any single piece of gear you use. Training, practice and time in the water.
Your stated lesson of learning to never rent gear, is NOT what you should be concentrating on.