I've been thinking about this kind of thing a lot recently.
Looking back, it seems like there were a lot of times when if I'd believed something untrue but optimistic, I might have been able to kickstart a virtuous cycle.
- If I believed my school/uni work was good enough I would have just submitted it, instead of procrastinating until I came up with something perfect, and submitting nothing, and failing.
- If I believed I was smart enough for my job, I would have been more motivated to work hard and ask for help, and less crippled by feelings of incompetence. Instead I thought about how stupid and incapable I was, and hid from everyone and never improved, and anxiously waited to be fired.
- If I believed I was normal looking, I wouldn't have been ashamed to go to the gym or buy nice clothes, or experiment with different hairstyles, and I probably would have looked a lot better than when I started. When ordinary-looking people liked me I wouldn't have wondered what was wrong with them, and when pretty people paid attention to me maybe I wouldn't have got so stupidly obsessed. Instead I stayed weak, my clothes stayed cheap and worn, and my hair stayed messy. I turned down relationships that would have been healthy for me and I stayed obsessing over people who were unattainable and uninterested.
- If I believed my health issues were both solvable and nothing to be ashamed of, I would have addressed them more diligently, instead of giving up and letting them get worse.
The problem is that none of those beliefs were really true.
- I really wasn't a top-tier student and I'm really not smart or diligent enough for academia.
- I'm really not as smart as the vast majority of my coworkers.
- I really do have a plain face and a weirdly-shaped body, and a few strikingly abnormal features.
- I really do have health problems that most people my age don't, and which are not necessarily curable.
...and it was totally obvious to me that they weren't. It's only now, much later, that I can entertain the possibility that things might have been different, if only I'd acted differently. And maybe things can still even be different now (I doubt it), but I won't know until it's too late again.
Martin Seligman has written that depressed people tend to appraise reality more accurately than happy people, but that it is precisely this accuracy that leads them to take none of the chances that can lead to genuine happiness. He suggests (I'm very loosely paraphrasing) that the reason CBT works is not because it gets people to think more accurately about the world, but because it provides them a pathway into fooling themselves in productive ways.
I've mostly lived a very isolated life, but as I've got to know people a little better over the past few years it has shocked me how generous their appraisals of themselves are, how confident they can be despite having little to be confident about, and how often their attitudes end up paying off for them. As to whether it's possible or desirable to live that way... I don't know.