Hi Julian,
I am not sure how much I can help here, as I am coming from the opposite direction (pre-everything MtF) and only a few years into my marriage with no children yet. The only similarity I see is in trying to keep a marriage together while also wanting to transition, and my wife being mostly heterosexual.
My wife and I are materially independent, even of each other, and share a mostly equal relationship with none of the additional 'constraints' as in your case. So it is only our emotions and feelings towards each other which make us want to work everything out and avoid the prospect of being alone or trying to find someone else afresh. Like Adrian, I too go through these phases of wondering whether my wife 'loves me enough' to support me, but then I realize it sounds selfish because she could very well ask if I 'love her enough' to not transition.
I had initially thought (still think so, actually) that we could stay together even if I fully transition, but I am getting the sense that eventually, we may each have to make our own choices and not necessarily bind each other down. We are taking it one step at a time, still treating each other with as much love as we always have, but we know that at some point, my transitioning may bring matters to a head.
The reason I am saying all this is because even with a marriage which is working so well, I am wrestling with these feelings about my gender which may finally lead me to put everything at stake. But if for some reason, it wasn't working well and I had been feeling that I was holding things together more than my partner, it would probably make my decision about transitioning easier, and the fears about being alone lesser.
I know that this is easy to say at my stage of life, but not so easy to even consider later in life. After having been married for so long, and especially with the responsibility of children, you might feel comfortable or even obligated to continue as you always have and scared about suddenly being alone, but as Mrs Izzy rightly said, you might need to ask yourself whether you are being fair to yourself. From whatever you have shared, there is no reason why you can't do well for yourself and for your children even after transitioning. Being monogamous and codependent is a good thing, but probably not when the partner may not be willing to reciprocate the same way. And dating in (what may seem like) a small niche, or the prospect of being alone, or undesired physical changes may seem daunting now, but taking small steps towards your happiness can help you figure out whether those fears are for real, and if they are, then how best can you deal with them.
I hope this helps, and if it doesn't, please ignore. Or if it is inappropriate, I apologize.
Take care
Cindy
