Books on how to find happiness and contentment by yourself, so you don’t have to rely on others for said happiness?

I’ve been alone my entire life. Never had friends because I just couldn’t get past the hurdle of either opening myself up to them, or just being rejected for trying. And I get the feeling that as I get older (currently 28), things are only going to get harder, whether that’s making friends or finding a partner.

But everyone always says “You have to love yourself before you can love others”. Even though there’s not really much, if anything, about myself to love, I nevertheless do not want to reject what could be genuinely good advice.

So, barring therapy (can’t afford it), what books are out there that might help change my mindset or outlook on life and give me some happiness and contentment that I’m fine by myself? That I can be in my own company and not be miserable or lonely.