My heart is breaking... I thought Christians are suppose to love "unconditionally"

[Hmmm?]

There are very few xtians who will partner with an atheist.  The xtian belief system teaches that atheists will burn in hell for all eternity.  If your GF believes that, then she would constantly have to try to bring you back into the xtian fold, to save you from her god's eternal judgment for not believing he exists.  If she rejects you for your lack of belief, then it's not going to work.

My personal experience of xtian "unconditional love" is that it only applies to xtians who believe exactly the same things they do.  Everybody else on the planet can burn.

I've never thought of it that way.   I thought love means love and not only to the same believers as you are :/

2Co 6:14 Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness?

There's your problem.

My heart goes out to you, tragic, truly tragic.