I'd like to add the caution that not all depressed, romantically unsuccessful, or crisis-prone people are misery pits, particularly since many such people are likely to assume they themselves are. If you have friendships that extend for longer than a year or two, you are almost certainly not a misery pit.

From the perspective of a person who might be a misery pit: one key is to avoid overburdening any single person. Cycling between three or four support people can be helpful. For validation-seeking, I've found social media works well, since liking a sadpost is usually less stressful for most people than comforting a sad person. It's also helpful to prioritize learning how to create value for others. Try to have pleasant interactions with your support people (fandom, gaming, sharing jokes, talking politics, doing a hobby together, whatever) more often than you call on them for help. Consider learning a service-provision skill, such as cooking.

Things I've learned over the years related to this:

1. If someone argues with great force and convincingness that you shouldn't help them because they'll never get better, believe them.
2. Never, ever hire someone for a job they can't do as a "favor." This causes more trouble than simply giving them money.
3. You can be friends with someone who has serious life problems. (It would be awful and callous if you couldn't.) But you need to both acknowledge that you probably won't be able to singlehandedly fix their whole life, and you need to spend some of your time just about enjoying each other's company, independently from you trying to help them.
4. You probably can't, in the long term, be totally dependent on others to take care of you, if you're an adult; people will eventually just refuse to do that. This includes depending on emotional handholding. Even if you have a persistent problem like a lifelong mental illness that does require long-term help, you're eventually going to have to transition to "managing your problem responsibly" rather than being a perpetual damsel in distress. It's ultimately in your interest to become good at coping with life, despite how much better it can feel in the moment to be rescued.

Feels real and definitely reminds of me of some dynamics in which I've participated or that I have observed.

This is what causes people like Robert Greene (The 48 Laws of Power) to have a law that says "Infection: Avoid The Unhappy & Unlucky". Of course this is very cynical and I do not endorse it. Nevertheless, it seems like avoiding misery pits is a good idea, and I think one can tell after observing a person for a little while.

Tells: (1) Does the person make efforts? Are these efforts designed to make a change, or just for show? -- It can sometimes be hard to tell, but when you break it down into simpler and simpler directives, it may become very clear. I once had a person who would not follow simple instructions that a 5 years old could follow, that would have helped her to solve a class problem.

(2) Attitude. Does the person always make excuses for her failings? Or worse, non-excuses: self-flagellation, "I'm like this". Does she always have a reason why it won't work? Again, hard to tell. Sometimes, very much, if the person simply has a very different worldview. I do not recommend using this heuristic in general, but for potential misery pits it can be quite telling if the person has tons of problems and if for all of them there is learned helplessness.

That being said... take everything with a grain of salt (as you should always). For contrast, I was (and I guess still am) deeply in love with a misery pit who ended up leaving me after two years spent together. There is something there I don't quite understand, but I like to think that her redeeming qualities make up for the misery pit quality. But when all is said and done, I'm happy I didn't take Robert Greene's advice. But this was also not "another stranger on the internet".

Thinking on some defensive (perhaps appropriately) reactions to this, I worry that it makes an attribution error in describing how some people are, as opposed to what some people do or some behaviors that have unintended impacts.

I don't honestly know if it's a helpful model, even purely descriptively, to say "some people are needy." In my experience, some people have more propensity than others to attention- and validation-seeking behaviors, but it varies a lot both contextually and over time for most cases I've seen.

Framing it instead as "some people seek more emotional validation than you're willing/able to give" may or may not be more correct, but it does have the advantage of highlighting similarities with other types of suffering and poverty (including the similarity that it's very hard to distinguish environment vs agency in the victims).