Would not consider myself a veteran, but I do have a couple of thoughts for you.
As far as physical limits are concerned, sometimes you really need to pushing through that stuff. You might have to do some endurance trainig out side of class...such as running, hiking, biking...etc. However, if you are dizzy or filling extremly light headed, take a break get some water and resume when your ready. With that said though, I think a big part of pushing thru physical limits, is to have a strong understanding of your mental limits. Don't let your mind talk your body out of improving its physical status. I will use the memory of summer-two a day football practice(not as hard core as marine training, but tough all the same). When doing laps aroud the field in the hot sun...I can remember thinking of so many excusses to stop running, but you press on and finish, and it pays off in the big game. Or in our case, on the mat during a seminar.
As for your second situation. Well as a married man, I'll just say this: that kind of thing is going to happen every now and then, even after your married....part of life and it goes both ways. Advice, well try to focus, leave your personal stuff off the mat...and if that doesn't help, take ukemi for an entire class(works for me). If nothing else, you can work out some frustration. Nothing like break falls to clear the mind and the sinuses:-). With all joking aside, it is imporant to have a clear mind before steping on the mat, so maybe try some focus meditation or some ki exercises to release that bottled up mental energy.
Good luck and train hard.