Why would 'feeling super happy' be the relevant, emotionally truthful or required reaction? It isn't.
Feel general goodwill towards them, wish them well, mean that - but only that.
Of course you still grieve and miss your mother. That is your grief, not his. His behaviour has changed your perception of him, your closeness to him, your relationship. That's a fact. That's ok.
The mistake I see in your OP is an assumption (on whose part I'm not sure) that everyone in the family ought to experience things the same way, feel the same, respond the same way. That's an odd, socially and emotionally unintelligent thing to think.
You're different, adult people with different life experiences. Your relationships naturally change over time.
Your father has behaved in ways that make you less close now than you once were, or than he imagined you were / expected you to be. <shrug> so it is.