I feel that a lot of people might have missed the purpose behind some of this film.
A lot of the things that tell us the story were underlying. They're subtle because mental health isn't always noticeable. There's no conclusive ending because mental health does not have an end. Mental Illness doesn't just simply go away after you had an experience or adventure, or because someone has told you they love you. It doesn't happen like a lot of films show us. What I loved about A Real Pain was that it tells us the truth about this.
There were a lot of things characters Benji and David did that you would need to have been paying attention to and/or have experienced yourself to see what's happening. e.g. how much time Benji spent in the bathroom or the rubberband. That's just two examples out of many.
I think Eisenberg did a great job writing and directing this film. I always liked his comedy, but after seeing the thought he put into this, and how many little details he added to tell us about these characters, I've become a big fan. I hope to see him writing and directing again soon.