There are a couple of Wo Daos in the game, so don't worry about it. Anyway it only has 20 uses and usable by units that are more often than not doubling, so maybe you're getting a solid 15 fights out of that thing.
Also, going from fe7 to fe6 is a bit of a jump in difficulty since fe6 feels (to me, at least) the most recent interpretation of "classic" fire emblem. so don't feel too bad if you're getting a little outplayed. Fir is a good unit, but she definitely takes a bit more work than Rutger and CON problems aside you really only use her for waifu points or if you have an undying love for Bartre. fe6 also has a stupidly large roster, so don't feel bad if you miss out on a unit or two, or let someone die. Yeah it would be better if you lost e.g. Zealot instead of Fir, but you can still get a good gameplay experience even without this one unit lol.