In fear of stating the obvious, if you don't know what to do regarding hoof care, find someone that does!
You say you 'look after your other horses so know what to do' but if that only means maintaining reasonable feet, that can be a very different ballgame to correcting problems. For eg. As a rule, regardless of how much higher her heels are to where they 'should' be, you shouldn't just cut them down to 'ideal' parameters.
Re the lower hoof walls, sounds like they're peeling & splitting & yes, there probably is rot in there as well as mud. Diet, nutrition & environment(wet footing for eg) are all likely to play a part in the cause, as well as neglected hoofcare. I'd be treating it like any other kind of 'seedy toe' & cleaning/cutting out as much as necessary & treating it with an antiseptic until it can grow out.
If you would like to send some hoof pics(see link below in my signature) and more info on her management, diet, etc, we could give you some more specific advice.