First and foremost, I highly recommend reading "
Matchstick Theory" by Psarah Alice. That is the most apt description of energy levels for chronic pain that I've found. It also applies quite nicely to mental illness.
Now that that's out of the way... I have Depression and Anxiety. I also happen to be "High-Functioning" Autistic. I'm not an expert on Chronic Pain, so don't flood my comments with nonsense about that. I'm just basing this off of my own personal experiences.
As I said, I have Depression. I went undiagnosed for a very long time, because at the point in my life I was at, it was mild, and I could just power through the bad days. I was also younger and dumber back then. I didn't know much about self-care, I didn't know much about mental health stuff. I've learned since then.
One thing I've learned is that for me, emotionally, things can be quite like riding on an old school wooden roller coaster. Ups, downs, the occasional curve that rattles you to the core, but fairly predictable. Anymore, it's a baby coaster, with fairly tame ups and downs, and few to no curves. This is due to a weekend at a crisis stabilization center, medication, a lot of therapy, a medication change, a lot of introspection, learning to recognize my moods and symptoms, and learning coping skills.
Speaking of learning... Something that I've had to learn, which surprised me, is that mental illness is often accompanied by physical symptoms. For me, it manifests as fatigue and body pains. These can get more intense if I push myself too far, or if it's too hot, too cold, too humid, super sunny, or a combination. If I have to run errands in the summer (here in the Midwest, summers can be brutal during the day), I try to run them before it gets too hot, but I don't always have that option. When I don't have the option to do what I need to do during the cooler hours of the morning, I put myself at risk for what I've come to think of as a Crash.
Crashes are where my body just refuses to go on. I become extremely fatigued, I can't think right, I can't focus. Words are hard to get out, even in writing, which is usually the medium in which I am most articulate. My emotional state may be stable overall, but I get anxiety symptoms in the from of restlessness and twitchyness. My only option is to rest, to sleep and try and reset for the next day. Sometimes, it just takes one long rest to reset.
Sometimes, the Crash lingers for a second day. And sometimes, the Crash will trick me into thinking I've recovered, only to come back with a vengeance the next day, after I've gone and done something that takes a fair amount of energy.
Crashes are extremely frustrating for me. I have chores that need done, I have hobbies that I want to devote chunks of time to, I have jobs to apply for, I have apartments to research. When a Crash happens, all that has to go on pause. I don't like feeling like I'm not doing what I'm "supposed" to be doing. But Crashes are also important. They remind me that I need to remember to take time to practice self-care, in the form of resting. They remind me that I am human, and that as a human, I need to stop and rest, and not try to do more than I am capable of.
It just happens that what I'm capable of isn't the same amount of activity as I was when I was younger and not dealing with mental illness. But that's okay. I do what I can with what I have. I'm getting better at preventing Crashes, but I know I can't prevent them all. The best I can do is to try and take care of myself.