I've been moved from winger to full-back and I keep forgetting the player behind me

When we lose possession, the immediate priority is whether the central area is protected, but as a full-back, your role is to ensure that your 'hunt' doesn't leave you vulnerable to the blind-side runner. Your issue isn't speed; it is your point of reference before you commit forward. You are likely locking eyes on the ball while your team has possession, losing your scan of the winger behind you. Before we progress into the final third, establish a 'check-back' habit. Every time you receive the ball or even when we are circulating it, quickly glance over your shoulder to locate that winger. If you don't know where they are, you cannot recover effectively. Protect first, then hunt. Start by checking your shoulder every time the ball travels to the opposite wing. Improvement looks like you consistently showing your coach you know exactly where your direct opponent is before you decide to sprint up the pitch.