feel up to something

[Daruma]

feel up to sth to have the strength and energy to do or deal with sth: Do we have to go to the party? I really don’t feel up to it. [+ -ing] After the accident she didn’t feel up to driving.

I really don't feel up to it.
Can I say "I really am not up to it" instead?

After the accident she didn't feel up to driving.
Can I say "After the accident she wasn't up to driving" instead?

[wise owl]

Actually, there is a verb "to be up to smth.." and it means "To be equal to, or prepared for" (from an online dictionary).
For example: he is up to the business, or the emergency.
or, there is another meaning "To be engaged in; to purpose, with the idea of doing ill or mischief"
For example: " I don't know what he's up to."

So, as you see both verbs have a similar meaning.
In your first sentence I would say: "I am really not up to it"
In the second sentence I would leave it as you wrote it.