Just be honest with them. You don't have a shot at solving any of these problems unless you tell them what's wrong. This stuck out:
'During meetings, I always feel like a killjoy by attempting to be realistic and focused and reserved about the brand new idea they come up with every other week and that I need to get started on right when the meeting ends.'
I have a non-technical co-founder who does the same thing. If they don't even bother to become the slightest bit technical (which would help them understand how writing software works, and how long it actually takes) then they aren't co-founding material. You need to be firm with them. "No, we need to remain focused on building up the feature set we already have, and making it perfect. All the bugs in the existing code need to be fixed before we start adding features."
'During meetings, I always feel like a killjoy by attempting to be realistic and focused and reserved about the brand new idea they come up with every other week and that I need to get started on right when the meeting ends.'
I have a non-technical co-founder who does the same thing. If they don't even bother to become the slightest bit technical (which would help them understand how writing software works, and how long it actually takes) then they aren't co-founding material. You need to be firm with them. "No, we need to remain focused on building up the feature set we already have, and making it perfect. All the bugs in the existing code need to be fixed before we start adding features."