I'm not powerful enough to continue the main quest(s). What can I do?

It's seems I've hit a bump: Every time I try to battle something I die. I couldn't go to High Hrothgar because I couldn't beat the troll. So I figured I'd become a Stormcloak and head over to Windhelm and maybe level up on my journey there. So I leave Ivarstead, die to a bear, and end up fast traveling to Whiterun. I start off on the trail leading to Windhelm and die to a Frostbite Spider.

I'm a level 6 Breton. I have light armor and I am focused on 1H weps (mainly sword) and destruction magic (and a little alteration, but not til later in the game). Am I just advancing in the story line to quickly, or do I have a bad character build?

Bears are tough. A few tips: try upgrading your equipment if you have smithing, and either dual-wield swords (if that's your weapon of choice) and attack with both (both mouse buttons simultaneously on PD) or use a sword and a shield.

It's possible that it's just too early to do what you're trying to do. Level up a bit further before trying to progress the main storyline. Do a few side quests, get better weapons and armor, get a few perks in light armor and one-handed weapons.

Don't forget to bring potions!

The same thing happened to me the first time I tried to go to High Hrothgar. Really, you're at the point where you can either:

1. Buy a horse for 1000 gold (not easy at your level) and try to sprint up to High Hrothgar with it, or
2. Go do something else for a while. For me, I completed the College of Winterhold questline, and when I was done with it, I was level 13 or so, and at that point was able to make it to the top of High Hrothgar without dying.

The Main quest will always be there; and it levels up with you (the actual quest itself, not traveling to and fro), so you need not worry about completing it right now: You can always do other questlines for a few dozen levels (but don't wait until level 50 to do the main quest, because you'll find that it stops leveling up with you at that point).

Try gaining a few levels in smithing and alchemy. Both will pay them self really quick trough selling potion (Keep mana and health potions!) and weapons/armors. Investing a few perk points in smithing will grant you a huge advantage equipment wise.
This should also grant you a few level if you do it long enough ;)
Getting a companion (for example the mercenary in Whiterun) to tank some damage may help too.

Just run past. I did this at level 4. Good rewards at the top, too . . .