I just played really far into the game and got poisoned by a bug and mistakenly walked into a building with very little health. so as soon as i got in the buliding i died. and it saved me going into the building. so now every time it loads i die. so my guess is that i'm completely screwed and have to start all over again!!!!

Do you have any antivenom?

If its an auto save, load your manual save.
If its a manual save, open your pipboy and use antivenom and/or stimpaks.
If no stimpaks or antivenom and its a manual save, you're screwed.

It's generally a good idea to cycle through a few save slots so if something like this happens you don't loose more than half an hour or so. I also personally recommend that autosave always be disabled, it seems to be the root of a lot of problems.

What platform are you on? If you're playing on a PC and you can enter the console quickly enough you should be able to save your character. If you're on a console and you don't have any anti-venom or healing items on you you probably are out of luck - which certainly will svck if you don't have any other save slots for this character.

Do you have any other save files? If so, you can return to that save before the autosave.
If you don't have any other saves, unfortuneatly you will have to start the game over.
If you have any Antivenom, use it right after the autosave loads.

My last save was like at the beginning of the game. Looks like i'm completey screwed!!!

Rule 1 of Bethesda games: SAVE OFTEN!

I managed to pull up the Pip-Boy before i die finally. and took stimpacks till the posion was gone. so problem solved!!!!! and i'm gonna save now that i've learned my lesson

Actually. that would be a rule of any RPG or FPS or Sim game or Strategy game or any game really.

Nothing worse than getting really far and dying/failing/losing/crashing and realising you hadn't saved in ages.