I know they aren't really encouraged or reccomended but what about a nipple shield?
My DD2 was tongue tied and has quite a short tongue. She just could not latch on in the beginning and was losing weight. After resorting to expressing for feeds I reluctantly tried a nipple shield. I'd used them with DD1 for 5 months whilst topping up with formula which depleted my supply.
16 weeks on we are still using them now (mam brand are the best i found-tho I had to order from oseas after being given one in hospital)
I have been trying to wean her off it and I'm sure I could if I tried harder. She can latch on but starts fussing about. I'm lucky my supply has not been affected and we're still going strong.