Very often when I am concentrating on my heart centre during meditation, I find that my breathing is very heavy and distracting.
In your case, you are straining your eyes while you are meditating. Your eyes are tight and stiff, and the pressure falls on your heart centre. This is very harmful to your aspiration. If you can keep your eyes normal and relaxed while you are meditating, this pressure will go away. You can keep your eyes open or closed; only don't make them unnaturally stiff.