In someways I get the sense that you're looking to us to tell you to keep trying or don't give up or looking for us to somehow give you the solution to fix the problem the way you want it fixed. I "think" that is what you really want to hear, but I could be wrong.

Now for a more logical response... In DBT a part of interpersonal relationships is ending hopeless relationships. I would recommend is to take a look at pros/cons. The pros/cons of having Robert as a friend.. and the pros/cons of not having Robert as a friend. Write down everything no matter how insignificant you think it might be.. I think this will give you some insight on how you should proceed.

If you decide it's best to end the friendship.. It doesn't mean circumstances can't change one day in the future. When I say future.. I mean WAY off in the future.. If you choose to end it, go no contact and move on.

I hope this helps a little.