I think that I have watered it way too much over the last few months and that it got some kind of root rot. It started dying off from the bottom like 2 weeks ago. Today it broke off in the middle.
Does anyone have any idea how to save my little green friend? Or is it too late?
You’re good! Succulents are easy to propagate from stems, just leave it out of the soil and direct sunlight long enough for the wound on the stem to callus over (2-3 days after cutting, already done in your case), then plant it in dry soil and wait a couple of weeks for new roots to start growing before watering. Some of the lower leaves may shrivel and drop off, but the rest of the plant will be fine once the stem grows new roots.
That leaf that fell off might be viable still as well, just let the wound callus over before planting it in the soil.