I'm a huge sappy individual, but was what I wrote:

"This might be a bit awkward, but you are the closest thing I have to a father and I hope you have wonderful father’s day. Thank you for all the time and energy you have spent with me. Your presence in my life has shown me that men can be reliable, stable and kind. You make me feel like I’m worth something and I'm grateful to have you as my therapist.

Love S

P.s I don’t want a reply back"