I can't seem to hit high notes on the French Horn?

I am not a fan of telling people to just change their embouchure but you have a big issue and you admit that you think your bad embouchure is the culprit. Unfortunately there is not a single "correct" embouchure that I can say do that and your playing will be fixed. You have to find one that fits your facial structure that allows you to play comfortably anything that you need to do, but it sounds like what you are doing now is not working for you. Playing slightly to one side is not normally much of an issue, as it actually helps some people. I would suggest finding a qualified horn teacher to help you figure out what embouchure is best for you and to help you make that transition.