relationship trouble ... how do I decide? Do i have to decide?

For the past 6 years i have been in a long term relationship with a man who I love very much, 9 months ago I met another man at work who i was instantly attracted to, I started an affair with this man and eventually told my long term partner and moved out of our shared home in to a place on my own, after an unplanned pregnancy and unfortunate miscarrage I ended things with the man from work ... spent a lot of time alone and began to rekindle a friendship with my long term partner, which eventually turned back in to a relationship ... all was going well (although not muck 'spark') until the man from work contacted me to meet for dinner, I went, and have never experienced such strong feelings towards someone, i had butterflies for the first time in years and can't stop thinking about him ... I really don't know what to do? Do i try again with the man from work? Stick with the long term partner and hope the spark returns? See both? see niether?

I still live alone and don't work in the same place anymore ...