As others have said, considering her threat, you'd best be really, REALLY sure of who you are and where you're heading before you tell her anything. And a gender therapist is probably you're best bet for figuring that out. Maybe you'll discover that if you had the freedom to wear the lingerie whenever you want, you wouldn't feel a need to transition... and in time, she might be OK with it.
Deception though, such as lying about getting rid of the clothes, or even seeing a therapist behind her back, will inevitably come back to haunt you. You'll ruin whatever sense of trust she may still have.
IMHO, she deserves to know the truth about you so she can make an informed decision about the rest of HER life too. Just be sure you know what that truth is first.
~Kate~