Sorry to hear you're having trouble. I had to ride mine for some months before it was right for me. But I'm skinny.
I wonder if perhaps when the saddle was level, you were sitting too far forward on the saddle? The horn then does some of the supporting job, no good obviously. This situation might arise when the saddle as a whole is too far back, requiring you to sit forward to reach the bars. Is is vital your sit bones rest on the wider part - because the Brooks is so firm, you should be able to feel this quite distinctly. Having a large behind should not have too much of an effect I would think - sit bones are where they are, regardless of the amount of padding around them.
Try again with the saddle level, experiment with the forward-rearward position until you feel the sit bones doing their job. Other bits should be relatively unpressured. Then once you have this right, have a go at holding the handle bars by leaning your trunk forward instead of slipping forward on the seat.
If you feel there is too much pressure on your hands, then the bars should be higher or closer. Paradoxically, the saddle might have to move backwards to get your legs to do their proper supporting job.
Check Peter White Cycles site - he writes a mean essay on fitting if you need more.