Dick couldn't attend Jason's funeral because he was on a space mission with the Titans, something he deeply regrets every day. He failed to protect Jason, he wasn't there for him, he was an idiot to him until a few months ago, before his death, he couldn't say goodbye to him.
He didn't take care of him, Bruce failed, Dick failed him in every way.
After his argument with Bruce for not telling him that Jason had died, Dick went to the cemetery and cried on top of Jason's tombstone, lamenting and blaming Bruce and blaming himself.
He never forgot Jason, how could she? His memory destroyed him and haunted him, as if his ghost would not leave him alone. He wanted to kill the Joker, but the best he could do was beat him until he bled. He didn't speak to Bruce until Tim came into their lives and became Robin and they were slowly able to mend their relationship.
His romantic relationships didn't work out, not with Kory, not with Babs, not with Helena, not with Shawn. Dick always ended up failing them in some way... or he himself ended up thinking about Jason being with them.
Red Hood then appeared and Dick later learned that it was Jason, who had been revived in the Lazarus Pit. It was Jason, but not HIS Jason, no, this one was broken, more violent, resentful, not the Jason he knew. Jason was a criminal, a murderer, he tried to kill Tim and Dick was furious.
It was strange, Dick had very mixed feelings, sadness, guilt, anger, longing, all mixed together in no specific order, it was always like that with Jason. Jason eventually reached a truce/reconciliation with Bruce, even apologizing to Tim and began helping them on their missions on some occasions.
Jason treated him in a cold and sarcastic way and that hurt him. Dick made the effort to get closer, to understand him, but Jason's way of thinking is something that Dick will never be able to understand. And yet, Dick decided something: he was going to make it up to Jason for the rest of his life, with his own life and his own devotion.