I'm between a rock and a hard place... this June 6 I'll have a job... but today I went to visit the offices and discovered that my partner will be a woman... I'll be alone with her. ..for a long time... How will I keep from falling?
details of her: she is not Jewish, she is not religious, she is gentile and wears clothes of the world

Will it be a setup that is yichud? If yes, speak to your rav. If not, set up some gedarim. Hard to advuise without knowing specifics (is she Jewish/religious/respectful of religion/moral/etc.), but in some businesses guys can get away with not being on a first name basis with female workers. This sends a clear message that we have a working relationship, not a personal relationship.

details of her: she is not Jewish, she is not religious, she is gentile and wears clothes of the world

I had potential problems of yichud at work as I have to work often with a female coworker and was able to find a good workout with my Rabbi. In cases I thought there was no way out he helped me find good halachically sound solution, I would definitely recommend asking your rabbi. In terms of conducting yourself at work, if you present the personality of "I'm here to work and to focus, I don't really small talk" then I think people understand and won't try to engage you beyond what is necessary conversation. And if they do, you can give a polite nod and actually not say anything back (even if that doesn't make total sense conversationally) and they will eventually get the picture.

I will be honest.

I think it is a very dangerous position.

Even if you only talk about work and basic pleasantries, just being alone around her is too much temptation if she is dressing in that way.

Only keeping to work related conversation is not enough of a safeguard, and it is not realistic.
This is a colleague, and it is expected and polite to show some basic interest in her well being, without that extending into gossip or no Tzniusdik conversation. 
It is actually decent too.  People are not robots.

I would speak to your manager, explain that this is a difficult position for a Jew, and ask if there can be other people in the office.