Above advice is all execllent and should help you. For general interest I have a college who always talks about jumpers (and all athletes) having a strong leg and a clever leg. Your clever leg is the skillful one (eg the one you would kick a football with) your strong leg is normally the other side and is the one you normally jump off. So one problem with pistol squats is the high skill level – you clever leg can do it, but it’s more of a challenge for your weak leg. Having said that do the exercises about until you eventually do at least five pistol squats on both sides and I’m sure you will be ‘fixed’.