Q: Why is my Mac not recognizing my very correct password? How do I fix it?

My Macbook Pro has suddenly begun not recognizing my password. I have severely checked over the past few days and it is DEFINITELY the right password I am typing (my iCloud password), while my computer is saying it is not. Caps lock is not on. Nothing makes sense. At first, I logged into my computer with my normal password just fine, I tried to download a program and it wasn't accepting the password. I was confused, restarted the computer, and the same thing happened over again. I went to my settings to try to check and/or change the password, and my password didn't work there either! I restart my computer again, log in just fine, go to settings, and BAM suddenly my password works. I think to myself: maybe I was just crazy earlier. Since now I think it works, I try to download the program again...IT DOESN'T WORK AGAIN. I try settings again...it doesn't work again either. I forget about it for the rest of the day. Wake up the next morning and this time my password won't even let me into my computer. Trying to refrain from throwing my laptop across the room, I restart my computer, and the log in works just fine. I try to download the program again and, of course, my password does not work again. So I still cannot download the program I need, and it's for work so it is pretty inconvenient. Every one of these tries I typed my password twenty or so times trying it every way possible, slowly, really fast, caps on, caps off, making possible mistakes with certain letters if I had hit a wrong button...EVERYTHING. So, what is happening? And, more importantly, how can I fix it?? I saw a post about starting up your computer in single user mode and the things that post told me to type seem to not be working.