Do you control what you believe?

[Edwardo Piet]

Here's the way I see it:

You almost always believe something when you believe that there's sufficient evidence for it, but if your brain genuinely wants to ignore the evidence and be delusional it will. If you really really really wanted to believe that you're a snowman and you believed that your life depended on that belief and your will to live was extremely strong, your brain would force yourself to believe that delusion against the evidence in front of you. Reason is the slave of the passions.

So ultimately it's a matter of desire (usually in the form of fear, meaning you really want something to not happen), and the reason why it seems that it isn't is because it's normal to desire to believe what you think is true. And it's a big and important built-in habit to believe that the truth is important.

However despite it being a choice that doesn't mean it's a free choice. Despite it being willed it doesn't mean the will is free. It doesn't mean that there's free will.