I would go back to the original reason I went to the fitter. What were you looking for when you visited the fitter?

If you are in pain within 6 miles the bike fit is probably not right for you. If it just feels foreign then giving it a chance may be the right approach.

You say you feel like you are sliding forward on the bike. That is not right. You shouldn't have to hold yourself back on the seat with your arms and hands. Core fitness or whatever else people want to throw at you, the bike needs to fit your body and fitness level today. As your fitness level changes you then make slight changes to the bicycle.

The other thing to keep in mind is that bike fits are only suggestions. You are the one doing the riding.

Later,
HB