I'm sure the problem is that your pothos is rootbound and needs a 2" larger pot.* That is why is perks up when it's out in the rain.* Generally yellowing leaves with a houseplant mean that a plant is rootbound.* Leaves are expendable to a plant and they will shed the ones they can't support.* You will probably have to tease the roots a bit as they are probably winding around and around in a tight ball.* This is a tough plant, so you can massage the outer layer of roots to loosen them.* Here's how to repot.
You will also find that pothos do lose their lower leaves as they grow, so your only hope of keeping it full will be to start new plants from cuttings.* I do that with mine every few years to refresh it.* In the wild in the jungle this plant grows as a vine up trees, losing it's lower leaves as it grows.* Don't overwater this plant.* I often remember to water after it's wilted a bit.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.