How do you get rid of bitterness?

I was just thinking about how much time in my life I've spent at the city council building. It's quite strange actually, that no matter how many new faces i see when I visit my mom at work, I feel at home, a kind of nostalgia. Even though my mom is working in a completely different building than when I was young, it still smells the same, with the same furniture and it has the same atmosphere.
Council folk; they all have the same sense of humour, the same jovial outlook on life. And there's always that guy who whistles all the time!

My mom and dad have both worked at the council for most of my life and I suppose most of theirs.

When I was about 10, I would travel in to town with my dad during the school holidays (when it was his turn) and spend the whole day at work with him. I would pop down to the 10th floor every now and then to visit my mom and on my way back, I would pick up a cup of vending-machine-coffee. I would sit at the desk of my dad's office computer and draw on Windows Paint Shop and play Solitaire and try to figure out Minesweeper.

My dad thought that I was so clever, because I knew more about computers than he did. I could even change the desktop colours!
When I think of those times I think of my dad quite fondly, and there are many times like those.
But I can't help but remember the bad parts. Not that he was an abuser in any way or anything drastic like that, I mean, I've had a generally good life. But I just can't seem to get the bitter taste out of my mouth.
I know it's good to forgive AND forget, but I struggle with the latter.

All the missed birthdays; the reluctant asking for money for school fees, clothes and books; the awkward phase of him searching for a new wife; the constant breaking down of my love for my mom; all-that-drinking.
- it all adds up to one big fat lump of confusion for a 10 year old girl.

I forgot to call him on his birthday the other day (the 8th of November). I remembered the next day, but still didn't call.
I still haven't called.

Am I subconsciously punishing him for all the things he's done?
Or am I just as bad as he is?