Broke up with my girlfriend because of my declining mental health. Suggest me a book

As the title says, I need a book that will help me improve my mental health due to a breakup. Personally, nothing is going well. A month after the breakup, I still want to be with her. Became a huge crybaby realising that I lost someone who's gentle and patient with me.

Just want a book that will make me eventually knowledgeable about this issue; or maybe just want to know whether I should let it go or reconcile. Anything along those lines.