Well ive been playing the main questline for a while,when i figured out that i was on the last quest, right before mr. house sent to fight in that oh so epic battle in the finale... problem is i sided with mr.house, when i really wanted to side with the ncr, any way i could go back an fix this?

reload previous save and kill house

i did that, now what?

go to NCR embassy in the strip

The joy of a game with good replay value. Not much you can do, you must have sided with house without realizing it

I am pretty certain that it is too late. I accidentally went on without realizing I failed For The Republic, the NCR MQ. If you have it, you can still do it. If not, try again next time.

Sounds like you have a good reason to play the game again. This time armed with hard won wisdom.

No joke, the second playthrough was more fun for me because I knew what NOT to do.