I just always wondered, how God created Himself?
God didn't create Himself. Eternal means He always existed. "Before the mountains were brought forth, Or ever You had formed the earth and the world, Even from everlasting to everlasting, You are God" (Psalms 90:2).
Even unbelievers understand that something has to always exist. In evolution that something is matter. In that theory, the universe started with a bang from a super-condensed particle. Of course, the flaw is that they can't explain how non-thinking random events created an ordered universe.
Thus, for Christians to acknowledge that there is something that always existed is not an impossible stretch. In fact it is more logical because we see an intelligent being bringing order to the universe He created, which matches the facts better.