Your Senecio is a succulent. That means it should be kept in a small (nursery) pot with soil allowed to dry out halfway deep into the pot before watering. It also should be placed on your sunniest windowsill.
I know the pot looked small to you, but it should not have been repotted. The larger pot and additional soil mean it takes longer for the soil to dry out properly. That, plus your frequent watering have probably caused the roots to rot and they are no longer able to absorb water even though the water is plentiful.
Let the soil get very dry before you water again. If it does not perk up after the soil dries, then it is unlikely to recover, I am sorry to say.
