How do you let go of the fear of buying during a dip?

Every single time that we are at the peak of a bull run, I tell myself, "I should have bought way more over the last bear market."

But when I see the dip, I can't help but feel a little fear. My intellect says to go all in, but the fear keeps me from buying all I want.

How do you deal with this? Is there something you do to remind yourself that everything will, logically, be okay?