What's a life lesson you learned the hard way that you'd want to share with others to help them avoid the same mistake?

Seconded.

I had a mate who didn't speak with his parents, lost his job and left his partner.

Took him in, rent free for 6 months. Got him on his feet, he got a new job and 6 months later he left... With my 10 year relationship. And my cat (but to be fair my ex was a vet so it made sense that she took him)

I thought he was a friend.