I would not make up lies to your friends that you are constantly having to change and/or remember after you said it. Just say you're busy or not up to it.
But it is important if you are going to disclose that it's only to people you most definitely trust.

Also, nothing wrong with spending time to yourself. So long as you try to get out a little. Sometimes my only goal can be to shower and go the store.
Sometimes I go out with a friend or two that know me and we all have problems.

I know the few times I've disclosed, for the most part, that person usually understood either because they had the same issue or knew a family member who did.