Hi Mambo. This is a fairly common complaint. Before I forget, have you ever visited hussmanfitness.org ? It's a site dedicated totally to BFL by a physician who answers a gazillion of questions in lawmen's terms. Very good!
Here's what I would do concerning biceps curls (I'm assuming your gym doesn't have 17.5, 22.5 or 27.5 pound dumbbells):
Set #1 12 reps at 15# (should feel easy-just trying to get bloodflow to the working muscles)
Set #2 10 reps at 20# (should feel the muscles activating, but again not hard)
Set #3 8 reps at 25# (should feel like you at working pretty hard, but not failing)
Set #4 6 reps at 30 (should feel like there's no way you can complete more reps after 6 reps)
Set #5 as many reps at 25# while maintaining proper form, so if you get to #7 and you start swinging your body to get weight up, switch down to 20# dumbbells for remainder of reps.
Then finish up with a set of hammer curls @ maybe 15#.
OR (if the above was way too taxing or impossible to keep good form)
Set #1 12 reps at 10 or 12.5#
Set #2 10 reps at 15 or 17.5#
Set #3 8 reps at 20#
Set #4 6 reps at 25#
Set #5 12 reps at 20#
Then finish up with hammer curls @ maybe 15#.
Or you could scrap dumbbells for this exercise and use a straight bar on the cable column using both arms for assistance. Or try supinating biceps curls. See exrx.net for lots of ideas.
Now my opinion is you should spend most of your effort
on compound exercises which work more than one muscle group such as chest press, (works chest, shoulders and triceps), Shoulder press (works shoulders, triceps & back), Pullups or Lat pulldowns (lats, biceps, and many other stabilizing muscles). It's no wonder that by the time you hit isolation exercises which usually work just one muscle group such as biceps curls and triceps extension that you run out of steam. Those muscles have already done alot of work with the compound exercises you already did. So I don't think it's a big deal to adjust weights downward on biceps or triceps isolation exercises if you're running on empty. Just go down in weight if you're losing form.
This isn't to say that isolation exercises are useless but with compound exercises you'll burn many more calories and transfer that strength over to real life movement more than with biceps curls (which make for a nice pump if that's your goal
Hopefully this makes sense...long day.