I cannot shake the feeling that my whole life depends on one girl

From what you wrote it seems to me that your co-worker is a narcissistic person, I think your assessment is correct, she was just seeking for attention/admiration to boost her ego. It’s a nasty manipulation/game that narcissists do. Narcissists are usually extroverted, very likeable by everybody, everybody considers them nice, they seem to others as charismatic persons. But this is just a mask they have.

“she doesn't see other peoples emotions very well”. Narcissists are incapable for empathy for others, they care only for the inflated sense of their own self-importance. Women often have narcissistic traits related to their appearance but your co-worker must be a full-blown narcissist.

We avoidants have usually low self-confidence and we are easy prey to narcissists. We crave for being accepted by others and when a seemingly charismatic person pays attention to us we easily idealize her/him. Narcissists know this and take advantage of this. A seemingly “perfect” person who shows extreme liking to us should raise our red flags that we may be dealing with a narcissist.

Sometimes avoidants can be overly friendly with others (as AzulOscuro said) perhaps in an attempt to overcompensate for being socially awkward or trying too hard to be nice so that others would like them. But avoidants generally have empathy for others. This is different from how narcissists behave.

This crossed my mind, but she doesn't crave admiration or doesn't try to be "larger than life". She can sometimes make jokes at her expense. I remember her saying that she was chubby before and that she could see it when looking at old pictures. The only situations that make her upset is looking stupid. She is also not THAT good looking, her personality and sweetness is what makes her look good. She dresses well, different clothes every day, not too much makeup, some nice jewelry, cute girlish voice.

The fact that I could be an easy prey for something "sinister" crossed my mind many times. It was really difficult while I was infatuated with her. I made a thread in a different forum a few weeks ago. I remembered it today and went there to read what I wrote about her, and my god it's full of paranoid thoughts and stories. The posts are full of desperation in an attempt to find a reason why this girl is acting that way toward me... "how can anyone like me, there must be some form of conspiracy behind this". Then in the end it all faded away and I accepted the fact that there may be something there. Only to realize that I am wrong.

Orvel, it's not that you are wrong. We don't know this girl. The thing is that we have extremely fixed thoughts and we can judge a person and being very wrong.
I don't think a person can be judge by a behaviour.

Yes, you are right. I was thinking about some "general traits" of narcissistic pd, but the truth is that they come in different forms.

One thing is clear. You like this girl psysically. She doesn't convince you as a person.
I was said by an old men. Follow your instinct. All of us, as much naive as we can be, have a natural instinct to read people and the reality. It's the natural defense mechanism.
I try to follow it so badly but I find very hard to hear it.

I don't know if it makes sense.

You are absolutely right. Physically I do like her. One of the best things about her are the eyes... now I realized that I don't even know what color are they.

While as a person... that illusion was broken.

Yes it makes sense. I guess I have always been obsessed with peoples behavior that I forgot about their personality.

Edit: It's important that I realized that the attraction is physical. Basically, a natures way of "forcing" mating. Now, I have to control myself to not give her more than I've given her so far.

Obviously my assumptions can be totally wrong, they are based on the limited information provided here and on my personal experience, you have of course the whole picture and you can assess way better and more accurately, sorry if I sounded too rigid in what I said.

If I have understood correctly you work in the same office with her? It might be difficult to keep her out of your mind if you like her (even if it is just in a physical way) and you have to be with her in the same place all the time.

The post where I said I only like her physically was made during the weekend... because I was angry and I tried to make myself not like her. So I actually like being around her, talking to her, looking at her... even despite her flaws, both physical and personality flaws. The only difference is I don't feel she is perfect like before or feel that impending doom I wrote about when I made this thread. She is just a girl... a girl I like.

She sits at the second desk behind me. About once a week she sits at the first one behind me. These are open desks without cubicles... so direct view at everything. When there is coffee break (twice a day) they all move near the table that's in front of me, while I stay at my desk. Before breakfast, me and her go to make it, just the two of us. During breakfast, four of us sit at a square table, she sits left of me.

I try to forget about her during the weekend... then Monday comes and BAM! I am again reminded of everything.

So yes... REALLY DIFFICULT!