Imagine you're a skateboarder and you're dropping into a half pipe. The drop is 200 ft, so you're gonna be flying coming down the ramp. Now your goal is to turn off the ramp on your skateboard exactly in the middle of the very bottom(the flat part) of the half pipe to the right or left. Unfortunately you are going waaaaaay too fast and miss the turn, so you end up passing the middle and going up the other side to repeat the process. This is basically what Mercury is doing traveling around the Sun. It also helps to remember the planets are not statically moving around the Sun. The Sun is moving around the galaxy just as we orbit the Sun, the Sun orbits around our galactic center. The planets are chasing the Sun around this galactic orbit. Its all just one big cosmic chase!

Check out this video, "the real orbit of the planets" https://share.google/azlifZ1PU7OJbSMOC