To be honest, you didn't make a dumb mistake. You may have made a beginner's mistake, but certainly not a dumb one.

A dumb mistake is to die, uninstall the game, come over here to whine about how unfair and unbalanced the game is and then remove it from your library to never play it again.

To add to some other posts,

- Set up your portal base in another biome, but close enough to the swamp to venture there easily. You could even consider to have it close to the coast, so you can dock your boat over there too.

- During the fight, you can retreat at the indistructable trees, but don't retreat in any nearby sunken crypt. Bonemass will be waithing there for you, most likely blocking the exit, making it impossible for you to get out or to get back inside.

- Put your weapons (mace, 2H hammer, shield, bow) and potions (health, stamina, anti poison) under a hot key. This gives you faster access to them compared to opening up your inventory, move your mouse pointer to what you need, click at it, close inventory again.
The latter could get you killed for sure if you're too slow.

- Practice dodge-rolling before even going in. Although running away is faster, dodge-rolling could occasionally help you too.

- Make sure you got the Eikthyr power available to be used.
Being a wet place, the Swamp is a pain for your stamina gain. Eikthyr's power will counterbalance that for some time; meaning less use of stamina potions.

- Even if you aren't able to craft the entire root armour set, make sure you got the helmet at least.
That one gives a bonus to poison resistance - not full resistance as some may think - which really helps against one of Bonemass and some of his spawn's attacks.

Good luck!

Thorin :)