It is very good that you are open and honest with your feelings. You are going to need to work this out if this relationship is going to work.
However... his grief for someone who died is very real. It is a part of who he is and she was clearly important to him. If you are going to be with him, you will need to accept this. It is not something that you can ask someone to just get over.
He can love her (as his past) and you (as his present) at the same time. There isn't a competition here... he can do both. You are both important parts of his life. Talking about this is good. You need to express the difficult feelings. But, also focus on the present and how you can make this relationship work for both of you. I think it is fair for you to ask that his relationship with you be special and unique just for you (and not just something from a past relationship).
Does he do things that make you feel special and important?