I think it is natural to feel bitter if you haven't entered into single parenthood by choice.
However it doesn't sound like this man is any great loss, and continuing to hold on to the resentment and negative feelings three years later is only hurting you, when he obviously doesn't give a toss.
My mum left my dad when I was little, and he is still harping on how hard done by he was nearly 40 years later.
He obviously didn't like being a single parent but could have picked himself up, dusted himself off and met someone else, maybe even someone with children already so I could have had siblings, but he didn't, he just wallowed.
Never even went on a date!
You are still in your prime with the whole rest of your life to look forward to. Don't be like my dad.
I am sure there are plenty of men who would love to have a relationship with you and be over the moon to participate in your children's lives.
It is not necessary to have provided the sperm to be a father.
Have you thought about dipping your toe back into dating?