Suggestions for fixing a bulky heel on felted slippers?

I have a pair of felted slippers, very similar to these.
The problem is that she puts too much space in the heel, so the rest of the slipper fits, but I have this bit of bulk that sticks straight off of the back of my heel… about an inch. It isn’t too big on the sole of the slipper, and it fits perfectly everywhere else. The heel just protrudes from my heel by about an inch. How would I go about fixing that? This is my second pair of these slippers and I love them, but they tend to flop off after a while because of that extra bulk.