1.) Does there ever come a point where it just isn't worth it to continue trying to be a software engineer who can get into top-tier companies / projects?

Maybe, though I don't think you're at that point yet. More specifically, though, you may do better to simply let go of this idea that you need to join a "big four" (or "top-tier" company in any other sense) in order to feel like you've reached your potential (or are on a safe track to it). Why? For one thing a lot of people who "make it" into those companies report back that (apart from the prestige) the experience just wasn't as rewarding or interesting as they thought it would be.

But even more fundamentally: After all, you have to remember that none of these companies were anything like what our image of them is now, back when they started. And they certainly weren't "the companies to go work for and make a name for yourself", back in their very early days.

Point being: rather than chasing after the coattails of what other people have done (and which society later deemed to be great), you may do better thinking up something you can do that will great, and make it your goal to bring it into reality. And by definition, its potential greatness may lie in the fact that it just isn't seen as "great" by a critical mass of people yet (or because it simply raw and unfinished, and waiting for someone to come along and provide fresh interpretation and perspective -- someone like you, perchance).

BTW, there's an obliquely related quote from Jamie Zawinski about running after "success", and where you end up as a result:

And there's another factor involved, which is that you can divide our industry into two kinds of people: those who want to go work for a company to make it successful, and those who want to go work for a successful company. Netscape's early success and rapid growth caused us to stop getting the former and start getting the latter.

https://www.jwz.org/gruntle/nomo.html

Things are rather different nowadays, and I wouldn't say that working for Google or FB now is anything like working for the ticking time-bomb that was Netscape in 1999. But, existentially, I think there's a similar lesson to be drawn from being obsessed about joining something "great" versus... doing something great.

And what's interesting about JWZ's is that, while he appears to have made out well enough for the years in the tech industry -- where he ultimately succeeded was in defining "success" on his own terms, even if it meant doing something completely different (running a music venue), even if was guaranteed not to make him rich or "influential" as certain other people with whom he also worked at Netscape, and who we hear a lot more about today.

2.) How can I find some positive reinforcement in interviewing / interview prep even if I constantly get rejected? I do perform post mortems on every interview in order to find areas to improve.

Fundamentally, we can only find positive reinforcement from within (and from helping others) -- not from other people's evaluations of us.

But in regard to interviews, it may help to remember that these are largely bullshit. Basically we're in the midst of a long-term speculative bubble in regard to the supposed potential of certain cargo-cult interviewing techniques (hashed out in the past few years in folkloric fashion) to assess people's inner qualities, and predict their potential for "greatness." Which basically seem to operate on the principle of, "Well we asked such-and-such questions before. So let's ask 5x as many, 10x harder. That'll get us people 50x better."

All of which are fundamentally, hopelessly flawed: not (just) because the questions are silly, and increasingly have become tests of rote memorization.

But because you don't do great things by "being" great, or simply by finding "great" people. Great things are accomplished through great ideas, and from the courage (and strength) to pursue them. And because you're pursuing something you believe in -- not some random goal that someone else put in front of you.

Focus on these qualities, and you'll have a much better shot and ending up where you really want to be in this life.