Chances are, like you've said, you've forgotten the variations of your password.
It is possible that you have had all your accounts compromised, but, unless you're dealing with someone especially malicious and mischievous, they probably didn't go through the trouble of changing all your passwords for two reasons (at least in my mind):
The time and effort required to do this
You would quickly realize something is amiss when you can't log into
the majority of your accounts; generally a malicious user wouldn't want to be discovered, so they wouldn't change your password in the first place.
So what is next regardless of being hacked? The true first step is admitting you have a password problem. We're all friends here, don't be embarrassed.
The next step is multiple steps, but TLDR; Get yourself a password manager. They are the best way to stay secure and have a variety of strong passwords for each account that you don't need to remember. You only have to remember your master password.
Figure out what password manager you want to use. There's plenty of options out there. Lastpass is a popular option. Keepass if you prefer something open source. There are plenty of options for this step. Some password managers have phone apps as well, which makes it easy to lookup your password when you're not at your computer/you don't want to add the browser extension to the computer you are using. Do a little research if you're feeling scholastic.
Think up a REALLY good master password. One that you can remember. XKCD style passwords, IMHO, are perfect, because they add a fun-load of entropy, and they are easier to remember than randomly generated passwords. Here is some OWASP material on what you're password should be comprised of if you're feeling really policy-oriented.
If you can't get into your accounts, reset the passwords like you have been. It'll be a pain the neck, but its better than throwing random passwords at the wall until one sticks.
Step 4 (AKA the grind):
Now its time to get rid of that common/slightly modified password of yours! Each password manager will have an option to create a randomly generated password. Create NEW randomly generated passwords and change your passwords for every online account you have.
Step 5 (depends on the manager you choose):
Add your passwords to the password manager. Either input them into the actual manager, or my preferred solution, log into ALL of your accounts in order to add the passwords to your password manager.
This, obviously, will not guarantee your safety online. But this makes it MUCH harder to access your accounts. Hopefully hard enough that it's not worth the hassle to get into your accounts. Hope this wasn't all stuff you already knew and it was at least mildly helpful!