Maybe keeping your hand on the wheel when you stop to keep it from turning backwards.
I did put a new belt on it before I started using it because the old one was rotted. I don’t know what effect a worn belt could have.
If yours is constantly reversing every time you slow to a stop, no wonder it is giving you fits. That reminds me of driving my old volkswagen beatle with a stick shift and bad idler. It died every time the RPM’s dropped. I had to keep gunning the engine for a drive around a quiet neighborhood. Not a lot of fun, for me or the neighbors. At least it didn’t backfire.
It’s not mine. It belonged to the school system where I went to school in the 1940s, so it might have been a very old treadle model. I don’t think there was anything wrong with it. Maybe the earliest ones did not have that device to keep it from going backwards.
Wish my father were still alive so he could explain it to us. He actually fixed sewing machines for a living—in a huge factory where they ran them at top speed for high production rates and consequently they were always breaking down.