Hello there. My mom is merely 50 years ols, but for the past two or so years, she's gradually gotten less and less motivated to do anything. This was difficult to notice due to how slowly it's progressed, but I know for sure she's changed because she doesn't water the plants anymore, and they use to be so important to her. The litter boxes are often full now before she gets around to changing them, and the cats actually run out of food and water often. That never happened in the past. Other chores are often left undone for weeks at a time, the house is a mess, and she never wants to get out of bed. I'm helping with the chores and I'm trying to motivate her to get out or at least sit up, but nothing works. She just yells at me to leave her alone. I can't even turn the light on without her snapping at me to turn it off. Today alone I have tried making her happy by playing her a song that cheers her up, putting silly makeup on, and throwing confetti on her. She smiled at these attempts but only for a second. I tried saying that she'll feel better if we go on a drive around town and get out and take pictures. Didn't work. Even tried a less morally correct tactic by asking her if she would rather have me stay with her in her room with the light on, or if she'd actually rather have the light off than me staying. I asked if she cared about me more or the light more. She stayed quiet. I was shocked.
She has never been diagnosed with anything, but of course I'd assume she's depressed. I just don't know what to do. (I am only 19).
I admire your attempts at cheering her up. I'm glad you at least got a smile out of her. But depression (or other mental health issues) can't be cured by cheer-up efforts. This is Not Her Fault and also not under her control. It most definitely is Not Your Fault. She has an undiagnosed medical problem that needs medical intervention.
The good news is that depression is treatable. In fact, most mental health issues are treatable to some extent. So whatever is wrong with Mom, the starting point to fixing it is to get it diagnosed.
You have perhaps heard of postpartum depression which some women experience after childbirth. Some women also experience menopausal depression. Could this apply to Mom?
But whatever the cause, the first step is to seek a diagnosis.
Does your mom have any friends or associates she might listen to more seriously than she would to you? A religious leader, a bowling buddy, a good friend? If so, explain the situation to them and ask if they would talk to her.