I'm with Longfins and Cliff on this.....take it easy. It's a wonderful past time that should be enjoyed. I love the challenge too, don't get me wrong, but try to forget that stuff sometimes. If you can dive to 35 feet for a minute or so, then relax as much as possible, and the minute will seem like like 5 minutes....Einstein was right!
When you're training for performance, spend some time diving in your comfort zone first....you will be relaxed, and enjoy your self. Look around slowly, and feel the cellular memory that most people never experience, even scuba divers.
After some comfort diving, then slowly move down the ladder. Until your lungs stretch enough to accept the depths you want to hit, you will feel the "go up" survival reaction. When I first started diving, -10 metres was agony; my chest was on fire, screaming at me that i had no air left, even though I did. Eventually that changed, and will for you too.
Most divers do not dive to -50' for 2 minutes, performance guys and spearos included. If you are getting 2 minutes of botom time, you are at the top of the curve. Most spearos are doing 45 seconds...of course there are exceptions, but they are not the norm.
I will generally go up as soon as I have reached 10 contractions when rec diving. I know I can do more, but then it gets dangerous. I would suggest that if you are at -50' and start getting contractions, that you gently return to the surface, gracefully and without angst.
Good luck, you are doing really very well,
Erik Y.