This is something I have grappled with. I think the context and environment is everything. It depends on the layout - how separate areas are, for example. And it depends on if you are stealth or not.
In my opinion, you shouldn't use the women's change room. You don't belong there - it's not fair to the women there. Whether you should use the men's room is really about your own comfort and the legal rights where you live. Eventually, you *should* be using the men's room, but I can understand not feeling comfortable doing so in a place you've been for a long time before transitioning and people know you.
For many years, I have used men's locker-rooms when I play hockey. Normally, the shower/bathroom area is separate from the changing area, so while I may use the bathroom before playing, I do not use or go into the shower/bathroom area afterwards. So I change to the extent I feel comfortable in the locker-room (not stripping all the way), and I don't care if there are men who are unclothed near me. My experience is that no one really cared even when I was viewed as female! Those who were more modest had alternatives (such as changing undergarments in the other area or under a towel) and some were more modest in general, no matter who was around.
Someday, I would like to be able to use the shower area too, after I have top surgery. I don't think that will ever happen for me at a place where I already have friends and am known, though. I don't think I'd ever be comfortable and I think other people who are otherwise cool with me would feel weird too. I dunno. I've managed with showering at home after for years, so it's not the biggest deal.
Basically, I think you should figure out why you feel like you can't use the men's room. Is it because you don't have privacy to fully strip? Could you use a stall or towel to do so? Is it because you think the other men are uncomfortable with it? I think this is one area that you'll just get used to doing, once it's not "weird" anymore, same as people have to get used to using men's bathrooms too.