Why is it so hard to act in accordance with our beliefs?

It is easier if you are around people who share similar beliefs and are acting on them. For example, some people who believe their calling as Christians is to help those in need might belong to churches that don't provide opportunities to exercise that kind of help. They need to find another church or organization that provides those opportunities to serve and associate with those people. it is easier to act according to your beliefs if you are with others who are acting on their similar beliefs.

Belief and Understand are learnings of the conscious mind.
There are Instincts, reflexes and prejudice, fears lurking in the subconscious !!
By acting on small things according to a belief one can condition oneself. And by regularly writing about your feelings and thoughts in details, one can identify the items lurking in subconscious and get rid of them. Slow process, mind you, but can be done.
Best wishes

I've always found when the conviction is strong, living by those principles was the most natural thing in the world.

I think it is easy. I see everyday how those who don't have "structure" and guidelines , struggle.

Bc its not your beliefs. Your actions reveal your beliefs and values

Depends on your beliefs. What are you struggling with?

Than something is missing

Idk.