You could, bring up a couple lingerie ideas, from the net, ask your bf, if any of them appeal to him, and whether he'd like to see you dress sexy?

Is the fear of rejection, from your current relationship, or a past one?

Part of being sexy, is exuding confidence, trusting your partner is all eyes, and won't reject you. (( you can even have little blunder, oops, turning an ankle in a high heel, but that's actually part of the intimacy))

Just 'own' the moment, when you decide to take that step