I have a mystery snail in with my Sven and he completely ignores it, doesn't even seem to notice it exists
on the other hand, I stuck it in with my other boy Robin one time to help get rid of some bits of a plant that melted, and Robin ended up with a bash on his forehead from trying to grab at the snail's antennae.
so yeah, any tank mates are going to be a risk with a betta.
also, a lot of the fish that can possibly work with bettas are schooling fish, which means you need to have several for them to be happy, and they generally need a decent amount of room, so they're not really recommended for anything under a 10 gallon. snails and shrimp are the ones that are generally recommended for smaller tanks, shrimp seem to have a pretty high chance of becoming lunch though.
as far as you doing something wrong, it's pretty hard to know without more info. size of tank, temperature, filtered, water change schedule, how much and how often you feed, live plants, etc. this
covers most of the basic stuff, but if you have any questions, please don't be afraid to ask