The idea of a guarantee is a myth.
When you apply for a job, you don’t know that you’ll get it.
When you start a marketing campaign, you don’t know that it’ll get traction.
When you ask for a favor, you don’t know know that someone will agree to it.
But you apply for the job, build the marketing campaign, and ask for the favor anyways.
Because in doing so, you create the best possible chance for the outcome you seek.
Stop trying to look for guarantees, you will not find them (okay, maybe your local fortune teller will have something to say about that).
If the work and change you seek to create matters enough, creating the best possible chance for it by trying is worth the effort alone.