The ethics of posting someone’s diary entry aside:
It reads to me like he fell in love with someone once (“not just a rock, but a mountain”), she broke up with him and he felt like the new guy won’t be able to love her as he does (“reduces herself to a rock in someone else’s collection of rocks”).
And this pain is what makes him both be himself around potential love interests (“When I became indifferent to it, I found myself sharing many beds”) and protect himself from being hurt agaim, by disassociating/erecting mental walls. (“I found myself performing, doing the right things”). The beginning about performance is just justifying the disassociation to himself as normal.
As to your question on how someone can be so happy alone? People usually aren’t, but if he’s single and never opens ups to anyone, they can prevent being hurt, at the cost of never really finding the same type of love again.
And how do you get over him?
Well, like him (I assume), you were hurt. Unlike him, you didn’t disassociate. Like him, you are stuck in the “What could I have done differently” (“If he makes a mistake […] it’s over.”) part of grieving.
To get over him, accept the pain your brain is trying to process. Feel the hurt and let yourself grief the relationship, whenever you need to.
And, eventually, try looking for the parts you love in your boyfriend. Know, that while he interpreted his subconscious wanting to put some effort into a relationship as a performance, you boyfriend likely doesn’t, but does it of his free will.
And delete, or at least lock away, the diary. There is no need to relive the pain, by reading the diary. In there isn’t anything that will change the past, nothing that will help you with your boyfriend and you can’t fix him, needs to accept and process his pain himself, too.
They didn’t thank Scribble (the dog) in their acknowledgements section. 1/10 paper, would only look at the contained dog picture