Confusion Over Intense Experience During Meditation
When stored accumulate stress dissolves, pure energy can rise up into the brain, causing this experience of "acceleration", "cleansing out of the mind" and "softening calmness". 

The "heart beating faster" and "heavy breaths" is both exhilaration and the beginnings of fear (since the mind has not experienced this before). 

Finally, the "fear" became strong and broken the meditation; because the mind could not handle the loss the control and the loss of self. 

Unless you have a history of mental illness, the experience should not be worried about. It was a perfectly normal &amp; ordinary meditation experience. If fact, the experience was a sign of progress (in terms of the calming &amp; purification process). 

When the body &amp; mind cleanse themselves in meditation of accumulated stresses, these "special effects" will happen and then subside. 

When the Buddha taught meditation, he provided the following basic instruction to be followed always; 99% of the time: 


  There is the case where a monk remains... ardent, clearly comprehending &amp; mindful — putting aside greed &amp; distress with reference to the world.