What I struggle with the most I think apart from normal sadness is that I don't know what is normal anymore. I was the only person in the world who cared when she was at her lowest. I was also the last person who should have. Most people would value that but of course to her it's nothing. I'm not even really sad this time I just feel empty. I knew it would happen but believed her again.
I felt the same and did the same things for my BPD partner for a long time in a similar situation many times. You might be there again as well as long as you have a small hope. For their part they mostly tell you what you want and need to hear and will promise anything. It does not last. The addiction usually causes them to lie more. Regaining trust is near impossible, but is what most normal relationships are built on.
My comment has more to do with WHAT IS NORMAL. I think you know what it should be and definitely what it is not. When NORMAL shows you'll recognize, appreciate and slip into it quite easily.