Re: Battery retaining charge... what am I doing wrong?
Could be a defective battery - they do that.
Could be you have something on all the time - easy way to tell - disconnect either battery cable, connect a test light from the disconnected cable to the post it just came off. The light should NOT glow at all, of it does, something's still on. Then it's a case of finding what that is. Shield it from light to see closely, sometimes they barely glow.
If the test light's off, have the battery load tested. It may even pass the load test though if you have it charged again. Sometimes they'll test fine, but not hold a charge overnight or for a longer time. The only way then is replace it.
Like me, it may hate the cold.