Basically, I went with the NCR for a bit until I went to the Legion. Deciding that the game was telling me to do everything the Legion told me, I went along with Ceaser until I was villified by the NCR.

Now I want to get back on their good side, but every time I approach them in the NCR embassy, they shoot at me.

Is there a way or should I just stick with the Legion for this playthrough and do the NCR on another?

Might as well stick with Legion. It seems very hard to get back on the good side of certain factions/people. You could try to find a person you can bribe to improve your reputation. I did that with freeside and got back my reputation to "Smiling Troublemaker".

you should've just ignore legion quest and stick with ncr quest

You might try wearing NCR uniforms to get close and then do quests for them to build rep back up. No idea if it will work, but I think it'd be fun to try to test the resilience of the faction system

P.S. Don't do what you think the game is telling you to do. Let quests fail! Look for other ways to do everything. Unless you want to do what you think the game is telling you to do, of course.

Yup, when you choose sides, you'll fail other factions' quests. Last nigh i pissed off Caesar for the last time, and got a flurry of Quest Failed messages

Disguise as NCR, do quests.

I walked into the NCR base wearing an NCR uniform. They still saw through my ruse and shot me full of hot lead. I read that I could possibly redeem myself by talking to the ambassador in the embassy in the northern-most part of the Strip, so I go there (still dodging bullets)...only to have the ambassador NOT be there.

So far, I've pissed off them and the Great Khans, I'm shunned by the Powder Gang, idolized by the boomers, and accepted by the leigon.

Know what? I'm going to create two more characters, one who will side with the NCR and the other will side with House.

Don't forget one more, yes man.

Even though I with Legion all fractions like NCR accept me but Powder Ganger they hate me to shreads.

Powder Ganger hates you after Gun fight in Ghost town; so it would be either get quest from them or keep the ability to edit your character after Gun fight in Ghost town.

I sided with the Legion at first, but I got bored of them one day and killed Caesar. Lol, so I guess you could do that to get on the NCR's good side....or just go up to the ambassador and get pardoned if you haven't already.

That's the thing. I can't find him. He's nonexistant. I run into the embassy under gunfire, run to where his office should be, yet he's not there.

I'm now vilified by the Legion since I am working for Yes Man.

Looks like my guy has managed to alienate virtually everyone except Yes Man.

Glad I made two other characters. Of course, I'll probably have to create a fourth one anyway.