I think working with a therapist with trans experience is a good start. I just recently told my therapist and found out that she has worked with other trans folks so scored a win. As for coming out, only my therapist and my wife know. And my wife was first. We have been together for 20 years, married for over 14 of them and I told her about 3-4 years ago. I don’t like holding secrets from her but this was something I buried deep inside and has been growing since coming out to myself about 5 years ago. I got very lucky that she is willing to be part of my discovery of what this all means. I would say giving your partner time to think over something that you have been thinking over for a very long time can be key. I would recommend reading She’s Not There by Jennifer Finney Boylan. I just finished it and found it very helpful.