You've already been given excellent advice by both Settra and herne_the_hunter, so I won't repeat that.
Another thing to ask yourself is whether you
always have trouble with end-of-dungeon bosses or not. If you only have trouble sometimes - if, for instance, you swatted the previous bandit chief like a fly, but the current bandit chief just will not fall over and die - you might only have a temporary problem caused by levelling. When you gain a level, you get to progress a little bit; you get to increase one ability a bit, and you get one perk point. Bandit chiefs don't increase in power every time you level up, but when they
do increase in power, they improve much more than you do in a single level. If you meet a bandit chief just after that power increase, it can come as a bit of a surprise. If you're having one of those moments, it's sometimes worth going away, gaining a few more levels, and then coming back.
(Here's the
uesp page on bandits, as an example of how bandit chief power levels change with your level. Other types of enemy level differently.)
To almost echo Settra, I'd also say that picking stuff up is a good plan. Don't pick up
everything, but do pick up potions and spell scrolls. And then use them. I had terrible trouble with the end boss of Bleak Falls Barrow before I finally realised I'd picked up a very useful spell scroll on the way through Bleak Falls...