By meditation alone and trying to concentrate you will never succeed. There must be an aspiration from the heart and a giving up of all yourself to Krishna.
In your nature there are many obstacles, chiefly a great activity of the outward-going mind and a thick crust of the impure lower Prakriti that covers the heart and the vital being. Quieting of the mind and purification of the nature are what you must have before you can fulfil your aim. Aspire for these two things first; ask for them constantly from above. You will not be able to achieve them by your own unaided effort.
Ref: Letters on Yoga – II