[Pansy-Mae]

Progesterone is usually started after being on HRT for several years. This is more similar to how a cis girl develops. The way I understand it, they don't start producing progesterone until just before they get their first period, but usually start producing E and growing breasts sooner. Starting too early may actually stop breasts from growing according to some things I've heard.

You mean starting progesterone too early can inhibit growing breasts.
Thanks!

but I think it's okay to take progesterone now because 
I've been on NBE for months.

Paula is correct. Progesterone acts to develop the milk producing structures of the breast and must have the basic breast structure in place to work on.
I have read that taking Prog too early can inhibit the PM/Herb phyto-estrogen  action, and given that those are much weaker than natural estrogens, it would make some sense.
I think it is irrelevant how long you have been on NBE, what matters is that you are well past the 'budding' stage and have significant breast growth already before trying progesterone to round out the probabaly 'triangular' initial growth.