Your pain of loss of your father sounds very powerful, and my guess is you don’t also want to go through the loss of another important man in your life = even though it is time to let your boyfriend go. As you continue your grieving I would allow yourself to be more truthful with your boyfriend and begin explaining that he doesn’t fulfill you as he once did, and that you are not willing to continually make the investment. Explain that it is becoming more and more difficult for you to continue the relationship. Be honest with him. My guess is that in allowing yourself to realize he isn’t what you need or want, he will be able to realize the relationship can’t be what he needs either.
Cutting off the relationship rather than letting it come to a natural ending most likely feels too abrupt for you. Allow yourself to grieve the loss of your father and this will eventually allow you to have the strength to let go of your boyfriend.