If you're here as a tourist then you're here to visit friends, relatives, see the sights, travel around the country, etc. You're not here to work, whether it be for a Swiss employer or a foreign one. If you're working however you do it, then you become a resident - not a tourist - and a) need permission to become a resident being a non-EU national and b) need to pay your taxes here because you're a resident of Switzerland.
Why is this so hard to understand? In the same way I can't turn up in America as a tourist and work neither can you do the same in Switzerland.
Your options are:
a) get a job with a Swiss employer
b) your girlfriend can try and get a concubine permit for you
c) marry said girlfriend