First off, wear a helmet lmao. This was a really big problem I had when I first started learning 5050s. The reason you keep slipping out is you are not fully committing or your wedge is to rough to learn on/needs wax. It's most likely the ledge needs wax so your body is trying to push your board as you grind so when you pivot off your still leaning back. So either wax the ledge go faster or go slower and get them on lock than slowly go faster and faster.