Is your doctrine of perseverance a contradiction?

There is no contradiction between saying that one must do what one will do. That's only a contradiction if you think that God is not sovereign and that we must not will. But biblically both of those go together.

When God saves us, he saves us forever. And Paul says, "I am sure that he who began a good work in you will complete it to the day of Christ" (Philippians 1:6). So he tells them, "You will endure."

But in Philippians 3:12 you read, "Not that I have already obtained this or am already perfect, but I press on to make it my own"—that is, "I must persevere." Then he adds, "for Christ Jesus has made me his own." So fulfillment of the must is secured in the will.

And back in 2:13 we see that "it is God who works in you, both to will and to work for his good pleasure." Here you have the paradox, if you want to call it that. God is at work in my fulfilling the new covenant promise that I will endure to the end, but my response to that must be that I "work out [my] own salvation with fear and trembling" (2:12).

This is not a contradiction because if I say, "Well, he's going to do it for me; I don't need to do it," then I am simply giving evidence that he is not in me. That's not a contradiction.

It is both must and will; and if we don't fulfill the must we are giving evidence of the fact that God is not in us, and the promise that we will persevere doesn't apply to us.