If you ask me, the best reason to think "life is meaningful" is because one's life seems meaningful. If you can't stop "acting as if my own life had meaning," it's probably because it does have meaning. Indeed, not being able to stop acting as if one's life is meaningful is probably what it means for life to be meaningful.
Ross Douthat tweaks Wilkinson's argument. To make his point, Ross thinks about soldiers who keep fighting because "they know the war has meaning because they can’t stop acting like it has meaning":
In the context of the war, of course the battle feels meaningful. In the context of daily life as we experience it, of course our joys and sorrows feel intensely meaningful. But just as it surely makes a (if you will) meaningful difference why the war itself is being waged, it surely makes a rather large difference whether our joys and sorrows take place in, say, C.S. Lewis’s Christian universe or Richard Dawkins’s godless cosmos. Saying that “we know life is meaningful because it feels meaningful” is true for the first level of context, but non-responsive for the second.