I don't understand why you feel the need to read this to the new T... you can change Ts just because- you don't need to have a valid reason (many of those are BTW)/ the new T's approval.
Do you need the new T to say it's OK to leave that T?
Do you feel you can't/shouldn't otherwise?
Plus I read a lot of what he/you did and not much how you felt- maybe it'd be more beneficial to discuss that? Just discuss, no need to read a letter?