You invest into what you believe in. This spans all of life. If you believe your marriage is important, then you will invest in it. If you believe your local church is important, then you will invest in it. If you believe this friendship or that friendship is important, then you will invest in it.
The same is true for yourself.
Believe in yourself. Get up early. Exercise. Eat healthy. Laugh. Get in the Word. Audit a course. Take an online class. Podcast. Read. Find a mentor. Read articles. Get in debates. Watch the news (even if it makes you vomit). Get that degree. Take the risk. Travel. Broaden your worldview.
You prove that you believe in yourself by how you invest in yourself.