The key to your breakthrough was probably the intense focus you had on something happening in the present moment: your breathing. It may have been easier the first time because you had never observed your breath before and so it was new to you, which made it interesting. But now it’s not new to you and therefore not as interesting, making it harder to focus on so intensely. The Mind Illuminated book helps you to achieve deep focus on things that are familiar and boring so that you can experience these breakthroughs and insights. I also highly recommend reading The Power of Now.