First, I'll question some of the assumptions.
I do think there aren't enough leaders. (I'm happy to discuss whether Ender Wiggins was a good leader in comments.) Part of what makes someone a good leader is being able to recruit and work with other people, not just rationalists. In fact, I think for most rationalist projects, you need a rationalist leader, but not necessarily rationalist helpers. For example, Arbital didn't fail because we didn't have enough rationalists (we had 3/3), but because neither of us knew how to run that kind of a startup. However, Arbital very likely wouldn't have even happened unless I tried. And I'm pretty sure of that because after Eliezer approach me with the idea and I told him "no", six months later nobody was doing it. So I picked it up.
So, I think there are a lot of valuable projects out there that have nobody to do them. And if you look at most failed projects, rationalist or not, they don't fail because there weren't enough helpers, they fail because the person who was supposed to be the leader didn't do a good job.
Over the years I have collected a large pile of various failed or aborted projects in my wake. Things that just didn't work out or didn't go the way I had planned or hoped.
Yeah, welcome to the club. I think most people, even those who have been pretty successful have a long list of failed projects as well. If I understand you correctly, it seems like all of yours failed. I think that's still fine. Most (if not all, depending on how you count) of my projects failed too. I don't think that alone is a good enough reason to stop trying, though obviously you should adjust your strategy. (More on that in the next parts.)
Second, I'll give what I think is a helpful context to approach this from.
To me this question reads loosely as "Hmm, I'm trying things, but they aren't working out. May be the root problem is this one particular underlying assumption." And the umbrella question is "What should I do with my life?" As such, I think any answer has to feel right for you at multiple levels. (Jordan Peterson covers this idea very well.) So I'd caution anyone against hyper-focusing on one assumption / problem when it comes to answering a big question like that.
That said, if you ask yourself "What should I do with my life that feels right for me at all levels?", you often don't get an answer. So it's much better to go try something than not do anything at all. I guess, overall, I'd encourage people to approach this question from a more S1 / holistic feeling rather than S2 / systemic debugging.
I'm not sure this applies in your context, but it might apply to some readers, and it's an important enough point that I'll mention it anyway: when you're young, you're just not that capable. It takes a lot of learning and a lot of mistakes to get to the sweet spot. Some of the most helpful advice I've gotten was: "No, we don't want you here helping on this relatively unimportant project. Go out and learn and become stronger and better." It's a bit hard to swallow because it means going out there and failing. A bunch. But if you're the kind of person who can slog through that and learn, you're also likely the kind of person who can grow a tremendous amount because of that. And that seems a lot more valuable than being stuck helping on relatively unimportant projects, where in the end the project has moved forward, but you have been left behind.
Third, I'll share some of my relevant experience.
I actually have had a very similar question some years ago. "Hmm, Eliezer sure seems to produce a lot of good stuff. I bet if I could help him be even 20% more productive that would be a hell of a lot better than anything I could do." So I did. Around 2015, for about 3-4 months I helped Eliezer with a variety of things. I think it was marginally useful, may be even 20% if I stretch it. But it also became clear to me that it's not what I wanted to do. Even if that was the best I could ever hope to accomplish, that just wasn't a sustainable path. I'm italicizing that point because I think it's a very important realization and even at the risk of the typical mind fallacy, I do think it applies to basically everyone. For a path to be sustainable, you need for it to click on all levels. (Of course I didn't learn this lesson back then, it took another two "failures".)
When I worked on Arbital, I was the CEO. I didn't like most of the CEO-specific tasks like raising money. For my current project, I'm the CTO. And that feels much better for me. One could argue that the CEO is Daimyo and the CTO is the Samurai, but I think that really oversimplifies the relationship and much depends on the context. There're opportunities to lead and to follow, to teach and to listen. I'd say it's much closer to collaboration / interdependence than a hierarchy. The best thing is that I get to do the things I love (coding) and someone else gets to do the things they love (talking to investors).
Fourth, I'll answer the questions directly.
1) I think the question is not extremely useful. It's better to search for the intersection between what you enjoy, what you're good at, and what is valuable to do. Then do it in the best way you can given the situation. If that means leading because nobody is doing it, then do that. If it means joining an existing project, then do that. If it means convincing someone to do the project and then helping them or moving on, then do that.
2) I'd start by talking to people who know you and whom you trust. See in which directions they'd invite you to explore. Check with yourself to see what resonates. Give yourself a month or two to explore, but once exploration starts to slow down, just commit to the best task you've found.
3) I'm guessing that since you've asked these questions, that you are indeed very capable of making "a positive difference." And yes, I think you can improve it. Continue to aim high, learn, strive, accept failures, and do the best you can. I think that's all anyone can ask of you, including yourself. And with a bit of luck, in a decade or two, you're going to be a 1000 year old vampire capable of extraordinary feats.