How do I defend a message I don't believe without losing myself?

You shape the message. That is your job, and you're good at it. But some days the message isn't yours, and you defend it anyway. That gap — between what you say and what you believe — doesn't close itself. It sits there, and it charges rent. Here is the honest part: the compromise was never free. You just weren't counting the cost. Maybe you told yourself it was temporary, or that everyone does this, or that the skill matters more than the content. Those were ways of not looking directly at the trade you were making. This is not a question of whether to compromise. You already have, and likely will again. The real question is whether you know the price, whether you set it on purpose, and whether you're still willing to pay it. That is within your power. The rest is not.