If you can’t trust your therapist enough to tell them about it, then you need a new therapist.
The therapeutic relationship is one of mutual trust and understanding. You literally can not make any progress on therapeutic goals if this isn’t there.
What I would do is to share a lot of stuff that’s not the super-dangerous thing, make sure you have a real connection with them.
Once you have that, then you need to open up about it. If you can’t, then you’re just wasting your time there and need to either find a new therapist or a new way of working on yourself.