There are too parts to things getting better... one is you and one is him. You have no control over him doing better, treating you better, getting into a better relationship etc etc. And yes it is scary to think that this may go on for years....
You do have a big part in your dealing with it better, you finding ways to live your life and enjoy your life no matter how he is doing. It is hard but it can be done... i think there are several of us on this site you have learned to do this.
I will also say that my relationship with my son who is now 24 is better than it was. I am clearer about boundaries but I am also clearer in knowing that for me I have to continue to let him know I love him and I do... and he gets that. And for his part... he seems to be more interested for himself in recovery and that gives me hope.
And I have had a couple people in my life who were serious addicts who got clean and sober and so it is definitely possible... and one of them was an addict for years, did time in prison and got clean and gave a lot of help to others.