Help with boot

I got it yo work, and booted in, but then as soon as I restarted it, it asks me for the password (normal). Buuuut... as soon as I type my password in, it says incorrect. I know I type it in correct. Incorrect again! I restart it again, still have the same issue. This has happened to me 3 times now, and each time, I had to completely reinstall MacOS. Please help!!!