1) Learning about Attachment Theory was a breakthrough; it explains everything about my behavior, relationships, and life choices. It also suggests a method for change and ongoing personal development. Here are some books on Attachment Theory:

Attached https://www.amazon.com/Attached-Science-Adult-Attachment-You...

A General Theory of Love https://www.amazon.com/General-Theory-Love-Thomas-Lewis/dp/0...

Attachment Disturbances in Adults: Treatment for Comprehensive Repair [academic/professional text] https://www.amazon.com/Attachment-Disturbances-Adults-Treatm...

2) It's harder to make friends as you get older. Most friendships are built on an ongoing shared experience that is challenging, such as being in school together. Most of my adult-onset friendships are a result of having worked in small startup companies that were struggling to survive. Casual meetings of potential friends are much less likely to generate the energy and commitment for developing a friendship. If you're interested in the research on this, look into the work of Robin Dunbar, e.g.,

Friends: Understanding the Power of our Most Important Relationships https://www.amazon.com/Friends-Robin-Dunbar/dp/1408711737/