I was engaged to someone with kids when I came out. What did it was a daily struggle between being me and being with her, which was causing me to be so withdrawn that she thought I was in love with someone else. I ultimately decided that I could probably go the rest of my life without a transition just to be with her, but what would happen if she were gone and I suddenly found myself 60 years old, alone, and still in need of a transition? So I went ahead and told her.
She had dropped a lot of hints that she needed a man's man, so I was certain that this would end the relationship and my best case scenario was to keep her as a friend. To my surprise she was instantly accepting. That's not to say there haven't been any issues, but we talk about them and work them out. If it wasn't for her support, I'd still be considering the idea of HRT right now. As it turns out, I've been fulltime for a few months and on HRT for several more, much faster than I expected. Her, her friends, and her family built up a confidence in me that I never had before.
The biggest challenge has been the kids. The older ones don't address me directly anymore because they're still weirded out by it. The youngest took a couple months to come around with calling me the correct name, but she still has to get corrected on pronouns a lot. She also has a lot of questions that she never seems to feel comfortable asking in private. She asked me why I changed my name from deadname to newname while in a crowded restaurant. At another crowded restaurant, she asked me which bathroom I use. Another time she yelled across our apartment complex that I wasn't technically a girl yet, with lots of neighbors outside. She also still refuses to call me anything but her step-father when talking to other people.