I'm confused about my boobs.

I usually wear a 36D and it feels perfect but only at first(before it eventually stretches out) and I always have to wear it on the tightest hook, now I read somewhere that that's actually not good because it stretches the bra out faster, which is what I have experienced, so the next time I went bra shopping I went down in band size and tried on a 34D and it was WAY too tight even on the loosest hook, man I almost COULDNT BREATHE. The cups seemed a little small too. So now I'm really confused, should I just stick with 36D even if it does get too lose after a while or should I bare the suffering of a 34D until the bra stretches out??