The astral body naturally separates when the the mind, heart, and body are fully at rest and when we do not identify.
Any disturbance, physical or emotional, can snap one out of the transition phrase.
Do not get discouraged. We astral project every night, just without awareness. The key is not to get overexcited by the process, but to be conscious of it, to let the transition happen on its own.
Trial and error will lead to you astral projecting with consciousness more and more. When you get used to this natural process of nature, you can learn to explore the astral dimension with cognizance.
For thirty years I sought God. But when I looked carefully I found that in reality God was the seeker and I the sought. -Bayazid al-Bastami