Being able to shoot in the gym when no one is watching is a completely different feeling than shooting the ball with the game on the line in a packed arena. There isn't anything that can really simulate that type of pressure, but there are some basketball drills that you can use to help. The beat the pro drill is a shooting drill that puts pressure on the shooter and forces them to hit shots to win.
Beating this drill requires you to make one shot at a time, and sometimes you may find yourself in a hole where you have to dig your way out by hitting 5 or 6 shots in a row. After a while it doesn't become a big deal to hit a shot or multiple shots in a row with the drill on the line, and this translates into a game.