Fallout 3 ending question (big spoilers possible, obviously)

I beat fallout 3 earlier today and was very disappointed to find that my character cannot live if I do the final quest and complete it.

My question is, is there anything I can possibly do to go back and fix this. Its already innitiated, the giant robot shot up all the baddies and I can go to the memorial and finish it all whenever I feel, but I really want to continue and get all the side quests and a better ending if possible (for the love of god I hope its possible.)

I would say save the game frequently. Then go ahead and end the game to see how it goes, and then go back to a saved game and do the side quests with all your guns and money and have fun without ending the game. I think this game is meant to be an exploration of the entire world , and not just a mision to finish the main quest in the fastest time possible. I jsut finished the "American Dream" quest where I reported to the Citadel and got the armor and was asked to participate in the final fight, so I told them I need 1 minute. Now, I want to go colelct tons of weapons, and nukes so I can make the final fight spectacular. I also want to finish a lot of side quests and see how much equipment I can collect and store in my suite at TenPenny. Its cool, when I go back to places and see the destruction that I made there, and find there are new mutties to continue the fight, and they always bring more stimpacks and guns , so i pick them up easily now that I have Tesla armor and gattling laser gun.

I just wonder if the final fight will wait til I go back to the Citadel, or if things will move on, since they said time was critical, they had to move now. Wonder if the game will continue on even if I dont do the final quest right away.

But with so many different endings, the game was intended to be played multiple times. .IMAHAPYHIPPO