That's not quite the case - since all the relationships (by relationships I mean things we learn, things we experience, things we associate with one another) we invent are our-own, the only thing that matters is that they are not self-contradicting. Maths is a very good example of a system of tenets that holds up by itself (until you start quoting
Gödel that is
)
You are right, how do I know it works for me? Because somewhere you have to inject faith into the mix. Where you inject faith is up to you. How far you manage to go down the solipsistic rabbit-hole before it gets upsetting is up to you. Those who accept creation myths have probably not gone very far down that particular hole and have accepted something on trust. Some have a very different 'creation myth' that they call Evolution that they too have accepted on trust, its just that they've probably (but not necessarily) gone a little further into the hole first.
There are things we cannot know, it's been proven in multiple fields (by Gödel,
Turing,
Lorenz,
Heisenberg and
Einstein - among others) - and that may be the only true fact.