You do not need to diminish an achievement to make room for your disappointment.

Disappointment looks for a place to put itself, and the nearest available surface is often somebody else’s good news. So the mind starts editing: it was luck, the bar was low, they knew someone. That editing brings a moment of relief and then a small residue of shame, because you did not really believe it. Both things can be true at once. The achievement is real and your ache about your own position is also real.

Say the congratulation plainly, without the qualifier you were about to add. Then give the disappointment its own time later, where it belongs to you rather than to them.