Have you tried breastfeeding in a carrier like the hug-a-bub or otherwise a sling style?? I had 10E/F boobs and actually used to walk around the shops/house breastfeeding in one of these carriers. The hug-a-bub takes lots of positioning practice but is very discreet, the sling style is easier and quicker though.

Otherwise what always worked for me, was wearing those tiny bolero style jackets over any kind of top, which cover most of the top of the boob, even when you pull the whole thing out.