Home life

My boyfriend and I purchased a house together and have combined families, since living under one roof we have had many issues... but they all come down to his child (well adult child of 22) these things have been discussed with his child and nothing gets done about it... it is to the point now where we are doubting if we can live under one roof as a family. When things are discussed it is better for a few days then back to the same old.. almost like his kid doesn't care about anyone but herself.. I don't know what to do.. I feel so sick about this whole thing!!