Why is my succulent getting all mushy and rotting?

It's not frequent watering that ruins them - it's watering too much! Succulents like echeveria rot when the soil gets waterlogged, and that kills the roots. Let the soil dry out completely between waterings, even if a week or two passes, and switch to a pot with drainage holes if you don't have one. If it's already mushy, take it out of the pot, cut off the rotted parts with clean scissors, and repot it in really dry, sandy soil - sometimes you can still save it!

Echeveria in a pot with drainage holes is half the battle - if yours has a sealed bottom or drains poorly, switch it now. To save the one you have, take it out of the soil, cut off the black/mushy roots with clean scissors and repot it in well-draining substrate (a mix of soil with coarse sand or perlite). If the whole rosette is mushy, you can still propagate it: let the leaf dry for a few hours and place it on a plate with dry substrate, keeping it away from water until it develops roots.

The part that sometimes gets overlooked is the type of soil - if you're using regular potting soil, it holds way too much moisture and suffocates the roots. You need a well-draining mix, like succulent soil with coarse sand or perlite, otherwise the plant will rot even if you water it rarely. If the echeveria still has some firm sections, take it out of the pot, remove all the old soil, cut off the black roots with disinfected scissors, let it dry for 2-3 days, and repot it in a proper mix using a pot with drainage holes in the bottom. After that it can go weeks without watering - the succulent only wants water when the soil is completely dry and the pot feels light.

The thing nobody's mentioning is temperature and air humidity - if your plant is sitting in a really humid or stuffy corner, even if you water it sparingly, the roots still rot because evaporation's too slow. Echeveria needs air circulation and a well-ventilated spot, like near a window. If yours already has black roots, cut off all the soft parts, let it dry for a few hours, and repot it in really well-draining soil (mix up to 50% perlite or coarse sand), but this time put it somewhere with more air movement, or the problem'll come back.

The quickest way to save it is to take the plant out of the pot, let the roots air-dry for a day or two (the black and mushy ones should definitely be cut off), and then repot it in fresh, well-draining soil - like a mix of regular potting soil with sand or perlite. After that, just wait a really long time before watering again, like weeks, because succulents really do prefer being neglected.

I had a similar situation at home and what changed the game was stopping "regular" watering and switching to watering only when I see the soil is completely dried out and the leaves start to wrinkle a bit - then I water thoroughly and let it drain. I also moved the plant near a window with lots of light, because light really helps with metabolism and keeps everything healthier.