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Saying you're down to do something means the same thing as saying you're up for it.
(discuss.tchncs.de)
A "Showerthought" is a simple term used to describe the thoughts that pop into your head while you're doing everyday things like taking a shower, driving, or just daydreaming. A showerthought should offer a unique perspective on an ordinary part of life.
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.