Dude, reality check. The human brain is not perfect. It forgets all sorts of things. That’s just how it’s put together. Only a few freakish outliers have perfect memories, and even then it’s only for certain things. Unless neuroscience and genetic engineering advance way past anything we can imagine everyone will always forget things they’ve learned. And if that happened we wouldn’t be human any longer.
So study. Remember what you can. And don’t beat yourself up about it. This is the way the Lord made you. If it’s a sin to have an imperfect body then we might as well pack it up because everyone is going to gehennom.