You're not being unreasonable, but neither is your Dad. We all grieve in different ways, and unfortunately, our different needs don't always match.
The father of one of my closest friends has been widowed twice, and my friend said after the second death, that it was like his life had been on hold all those years - his father had been able to move on, but he hadn't, at least emotionally. He's doing his grieving now, and seeing a counsellor about some of his feelings.
Try not to stand in your father's way - he needs to take his own path through everything. It's okay to ask him for support, and for some time where it's just the two of you, but you need to let him have his new relationship (and don't forget your mother wanted him to start dating, too.) Try to recognise your own feelings and work through them, too - I don't think handling it all over 15 years later is ideal (though better late than never, and of course, he didn't realise what was going on at the time.)
I'm sorry for your loss - the one thing I've realised is you're never old enough to lose a parent. It is a hard time for everyone - be easy on yourself as well as on your father.