The worst part about having an adult child living at home is that he brings his childhood patterns with him or instantly falls back into them. You are never just a gracious couple sharing your home while he gets financially solvent, to be appreciated with gratitude for putting yourselves out. You are EVER the parent/ Bad Mommy who (once) said 'no' and now that he is an adult, HE will make his own decisions, thank you.
Variable #1: Because this person lived under your roof for 20 years, he believes it is his home - and it is, in some ways. Variable #2: Because he has not yet fully formed into a mature adult who understands life in all its complexities, he takes things in your home for granted that you have struggled to build. Variable #3: Because you once controlled his every move, and he has not yet realized that his every move impacts others, he will bristle at every reminder of this.
The minute we asked our son to leave (by the end of that weekend), after approximately 8 months back in our house, our relationship changed for the better. His chaos does not impact our lives. He does not constantly chafe at our boundaries. We are not pricked and poked at by his annoying rough edges. HE chooses to come by or not. And he is left dealing with the eddies caused by his own carelessness and unawareness of time, as it should be. And our relationships with him - both my husband's and mine (which I thought was almost irreparable) have improved dramatically.