Focus on what you want to get out of telling your T rather than his reaction? Why would it be helpful to you for you to tell him?

I would start with stating a basic "fact" or two but not be very descriptive to begin with, "My _______ abused me" would do fine for a start? Maybe mention you've never told anyone because _________ or, "I've always felt (it or I) was ___________ (however you feel).