I sit real far back on my saddle Asked this on diaspora, but was urged to come here, so, here's my question:

I tend to slide BACK on my saddle really far, to the point where my sit bones are very near to the edge. If I try to scoot forward, it's markedly more uncomfortable for the soft tissues and the angle of my legs doesn't feel right. The bike frame itself is the right size-- the shop helped me with that. So I don't know if I just have a big ass, a weirdly long torso, or what?

I was told that I should never, under any circumstance, adjust the seat further back on the post, but I'm somewhat at a loss of what I should do to fit my body's dimensions.

Also, I need a new saddle anyways, so any suggestions would be helpful. (Assuming they won't cost me an arm and a leg. I'm unemployed, after all.)
As others have said, just because the bike shop says it's a good fit, doesn't make it so. Their incentive is to sell a bike off the floor so they'll find the one that fits best and sell it to you.

I got a custom fit and I have longer thighs than most people. This meant that to get the seat position right, I had to have my saddle further back from the pedals than is typical. I ended up getting a frame built to my measurements.

It's possible you have the same setup. If you can't afford a bike fit, then look for a "setback seatpost" and you can do some trial and error.

The goal of having the seat in the right position for you is to allow your legs to cantilever your torso and "unweight" your hands, arms and shoulders. Here's an article that talks about the benefits of unweighting your arms and how far back to set your seat to achieve this.