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

Working on an early stage start-up now and everything that can go wrong, seem to go wrong. One co-founder recently bailed, another (part-time) co-founder isn't willing to go all-in. I've been blaming myself for this, particularly my judgment of who to work with.

How common is this? Would love to hear some thoughts and hopefully, encouragement!