As someone who is feeling like I'm starting to climb out of a multi-year burnout episode, I can tell you that you are definitely not a failure, even though it definitely does feel like it.
I believe I know exactly how you feel because up until about a few days ago, I also considered myself a failure, for similar reasons.
It's hard to compare apples to oranges of course, but in my case and just to give a bit of background, I quit my job while being part of team that created a very successful product for a Fortune 500 company ("failure" #1), my girlfriend left me 3 months after that, after 6+ years of relationship ("failure" #2), I felt my startup wasn't working and I felt trapped so I also left my startup, which didn't even had more than 3 customers ("failure" #3), meanwhile I pretty much lost all my savings ("failure" #4).
So, to sum up: no girlfriend, no money, no job, no startup... no nothing. Just 3 or 4 friends that endured while at my darkest moments.
After feeling like I failed at everything important in my life I felt lost. Like trying to sail the open sea during a cloudy night without navigation instruments, and not even knowing where was I supposed to port. I even felt so lost that I sometimes wasn't sure if I had a boat at all, or if there even was any sea left to navigate.
I know this is pretty "cliché" advice, but I can tell you that the feeling will go away at some point. It might take you yearS, but it will come. You just have to let all the experiences that you lived through settle down, so you can start seeing a path (or paths) in your life again.
Do not underestimate the amount of information you just got slammed with, that you haven't had the chance to process. And I don't mean only knowledge or skills, I mean emotionally.
Going through something as intense as having a company, and then (and this I can only imagine) having to sell it under such conditions, sets your brain and emotions into overdrive, just to be able to figure out what's going on, let alone to actually make anything out of it.
Don't push it. Just let it rest and things will start to get sorted in time. Of course, you still need to keep an eye open to avoid falling into a deep(er) depression, but other than that, I believe you just need to ride the wave. That's just part of the trip. And a necessary one at that.
In my case, I felt so disconnected to everyone else, because in my mind I was that guy that just can't make anything work, you know? "hey, look, he can't even keep a healthy relationship". "wow, that guy is such a failure, I mean, he just quit his job for some stupid dream! what a loser!", "incredible how stupid can some people be, right? I mean, who in his right mind would invest his life savings into such an stupid idea!?", and so on and so forth.
However, what you are not seeing (and will soon enough), is that after you process that boatload of experience, you will feel like king of the world. You might still be in the gutter (hopefully not, but it is possible), but you will feel like you at least were able to fought some of your most powerful inner demons and came out of it alive. Maybe you didn't beat the hell out of every demon, but you certainly punched more than one very hard and fast. And that feels fucking great once you realize what you just did.
You just got what I believe would be equivalent to a Master's degree. And I don't say that to be dismissive to people with actual degrees, but after the amount of stuff you had to do, what you had to prove to yourself and others, what you had to build (even without the slightest clue of how), I definitely consider it as a GREAT achievement, regardless of the "tangible" outcome (i.e. money, sales, etc).
Building things ex-nihilo is one of the hardest things I've experienced, but it also gives you such a perspective on the world that, even though I have no money, my personal relationships got strained and in some cases even broken, I have no job and I'm still in the process of getting job interviews, while at the same time having no money (and even a bit of debt), no savings, and pretty much nothing to show for what I did the last 4 years... I'd still do it again.
And I believe that after the dust settles, you will believe you'd do it all over again too (and you just might!).
So just hang in there. Trust me, this will pass and you will be much MUCH stronger and wise thanks to it.
I can even adventure to say that you will look back at this and remember it as one of your best experiences in your life. Not necessarily the most pleasant though, but one of the best nonetheless.
Cheers!