How we can break the core that mixed the body to the spirit . To travel out of body, I can feel my spirit but I am still stuck in my body .

Supposedly this not a safe thing to do unless you are protected because you are leaving your earthly body open to the entry of bad spirits.

Tickle is right. I have heard of people not being able to get back into their bodies. Don't know how true that is but try checking about Remote viewing and see if that requires leaving your body... it might be safer and less chance of bad spirits.

remote viewing (http://skepdic.com/remotevw.html)

I have heard of people not being able to get back into their bodies. So what happened to the bodies of the ones who couldn't return?

So what happened to the bodies of the ones who couldn't return?

I don't know but I heard riboflavin was the key to it. That might have been for time travel though. :p