I'm glad to read about your experiences because of how much your situation reminds me of my own. The anxiety, the worry, the fear of being somehow only a "second class" part of the relationship by virtue of being the newcomer, the uncertainty that comes of not having the same degree of connection to both partners...
It has helped me a lot to be brave and not hesitate to share my feelings and my "headbees." Every time I've finally managed to get up the nerve to share my insecurities with my partners (usually in person; once via email, when I was particularly unsure how to find the "right moment"!), I've been met with reassurance and support. I think that I've found that I just have to be okay with playing these things by ear; it's unrealistic to expect that my feelings for my girlfriend and for my boyfriend will progress in the same way or at the same pace, so I see no reason to try to force it.
I hope everything works out for you and your partners.