I can sell anything — so why don't I trust myself?

You can make almost anything sound worth doing. That is a real skill, built over years, and it works. The distrust you feel now is also real. Do not talk yourself out of it. Here is the honest shape of the problem. You have two separate acts: judging whether a thing deserves to exist, and building the argument that it does. For most of your career you did both, in order. Somewhere along the way, the second act got so fast and so good that it swallowed the first. Now you skip straight to persuasion, because you can, and you call the result conviction. This is within your power to fix, because judgment is yours. No client, no deadline, no fee controls whether you ask the question before you build the pitch. You have simply stopped asking it. Start again, today, with the next mandate in front of you.