Not to confuse you more, but with your phrasing you are correct.
If you’re up for it, or being up for something, you are interested. Similarly, if you’re down for something, or you’d be down for it, you are interested.
But if you are feeling down, you are not up for it.
The former 2, the verb is the action of being ready.
In the latter, the verb is feeling and down is the state.
For example, despite me feeling down I’m down to go out and party tonight.