How common is co-founder break-up at pre-launch startup?

I can't find the exact source, but I recall YC/Paul Graham saying that co-founders are the hardest part. Break ups early on are likely. It's so hard that it's probably the biggest factor an investor looks for; it's like how real estate investors look for location. You can change the type of building, but not the location. And you can pivot a startup, but not the founders. Hiring employees is hard, finding co-founders is harder.

My first business was done with a former best friend, the type I'd trust with my life. But these things happen. People have different values or priorities. And some don't commit fully. We had different values - he wanted a Ferrari and I wanted to create high quality jobs. He wanted something to boast about to friends, and I wanted something that wouldn't be a part of my identity.

Eventually met a great co-founder - he was a stranger I bought a book from via Facebook. When I asked him to be my co-founder, that was the second time I met him in person.

It's a lot like dating and marriage. Every mentor I met put it that way. It's a mix of luck and hard work that puts you in a position to be lucky. Some people get lucky early, some never, most never really figure it out. Eventually you just settle for someone who isn't perfect, and a big part of it working is to put in the effort to make it work.

It also only works if you're a giver and find someone who is similar. If you're the type who takes as much as you can (most people in business and elite jobs are), it's going to be really tough. There's just too much gray area, too many prisoner's dilemmas.

Very common, especially with friends. Everyone goes through these pains. It's easier to talk than to walk. Don't be so hard on yourself about it, just adapt and move on :)