I recognise so much of what you're saying.
I'm a '71 model myself, and I also felt dysphoric from the age of 5. Like you, I very soon learned to shut up about my gender issues when I faced constant criticism, correction and denial from my folks, not to mention the ridicule I experienced from my peers. I also threw myself into stereotypical activities to try to force myself to figure out how to be happy living as my birth sex; in my case, that involved marriage and childbirth. But none of it ever worked and my dysphoria increased with age. Then when I was 40 I finally got to the point you believe you're getting to: that point of no return where the stress of trying to live a lie has become overwhelming, and you know for certain that you simply can't carry on pretending to be something you're not.
Unfortunately, my experience with my family has been very different than Jill's. Not only is my husband very much heterosexual, he is also homophobic and, it turns out, transphobic too - so he made the decision that he wanted to end our sexual relationship and our marriage. We're still living together under the same roof to try to provide as stable an environment as possible for our teenage daughters, but it's an awkward arrangement. In the meantime, we're both finding it difficult to move on & find new partners whilst we're still living under the same roof.
So my advice to you is to explore this possibility in your preparatory sessions with your therapist, before you take your SO with you. I truly hope she'll respond positively, but it'd be helpful for you to prepare for the possibility that she might not.
I have to say though: even though coming out has cost me my marriage, several other relationships and my privacy... I've never been happier within myself than I am now that I'm living a more authentic life. Everybody deserves the right to be themselves.