My old worlds crash

My old (made in 1.8) world won't work in 18w15a. Actually, a snapshot or two ago, it crashed every time I opened it. I loaded an old save and worked up to the point it crashed (bred my horses, etc.). Now, I can't even open (in 18w15a) the old files I've loaded, as they crash every time I play! Even when I go one snapshot back from 18w15a they won't open, they just keep crashing.