Well for me I don't do a single underhook pass all that often by itself. It's kinda a gateway pass into other passes. Controlling their hips is important as with any pass, as is pressure and making them uncomfortable. Control of the opposite leg you are passing under has always seemed more important to me in that pass IMO.
McLeod, first of all I wish you and your young family the very best.
My immediate thought reading your initial post is are you talking no gi or gi? I'm 41, 4 stripes white in BJJ (2 1//2 years), and a brown belt in Judo (6 years). Most nights in class I'm 12-20 years older than the new guys, if not everyone. A lot of times newbs are extremely atheletic and powerful young men with impressive endurance. As a side note, I have a health issue that requires me to take a blood thinner daily, meaning that I can bruise very badly and bleed profusely if cut. Becuase of this, I've modeled my game into one of heavy top pressure and control of my opponents.
In our dojo, new guys almost always start out in no gi classes and stick with that until they decide to stay with BJJ. In no gi, I firmly beleive a new person can have a significant amount of success early on if they have solid physical attributes. I've been run over by some of these guys, and it can be a bit of an ego crush. That being said, it tests your skill and training, and forces me to find ways of dealing with a more powerful opponent.
In my game, it's when the new guys buy thier first gi that I get to turn the tables. I love to choke the shit out of them. The grips alone change the game because they slow them down and allow me to hold them much tighter in position. All kinds of sweeping and pinning options open up, and it negates some of their atheleticism. Anyway, I hope you find the solution to your situation.
Good luck sir!!
Good luck with the chemo.
Now moving on to the BJJ. I'm at a similar place with regards to size and BJJ (6'2" 220lbs. brown belt). When I roll with big, strong new guys (or any big, strong guy) I play a faster trapping game and focus on submissions rather than controlling and dominating. No sense controlling and dominating from the top and having to fight for things when you can outsmart someone and grab an easy sub. If all else fails grab something unexpected (wristlock, reverse armlock, collar choke, etc.). Good luck.
Again, appreciate all the kind words and input!