Meditation is the removal of “trying”. If you are trying, you are not meditating, not removing the minuscule efforts that keep you anxious. You are insufficiently depressed from perceptions of hope. The first time, although perhaps not even trying at all, your hope for success was probably minimum, if existent at all. You succeeded only because “you” didn’t really believe in your own effort, in your self, in your trying.
Apparently the first time it was something new, allowed in for sake of an avenue untaken, “giving it ALL up”. The second time held expectations, hopes for success in trying. A “part of you” was not giving up - the very demon that prevents you from your goal.
Your conscious is completely unaware of most of your inner self. The waters ripple, you know not from what. They still by means by your eyes unseen. You didn’t really think that it took all of those Buddhists practicing throughout their entire lives without cause, did you? Accomplishing the stillness of the waters is no small feat.
Not “You”.
You lack self-fidelity (aka “confidence”). When one lacks confidence what he succeeds at is not of his own choosing. Confidence is the state wherein the more significant influences within agree to the proposed effort, much like a well trained military regiment or a unanimous vote in Congress. There can be no disagreement in effort from within, no dissension in the ranks. The vote must be at least nearly unanimous. Why did you think all of those governments have always demanded that all dissenters be silenced and removed. Maximum effect, especially in stilling of the waters, requires maximum cooperation, whether for good or for bad.
Your initial mediation worked due to your lack of confidence in trying to do anything at all (subtle depression, no perception of hope). And your next effort failed because of your lack of confidence in not trying to do anything at all (too much perceived hope due to the prior success). You tried to hard because you wanted too desperately.
It is a classic case. In short, you are insufficiently whole, (aka “holy”).
To successfully meditate at will requires gaining an inner confidence toward at least that one effort, although more complete confidence would be much better. One cannot command oneself to be depressed from all hope. Although with proper belief, one can certainly convince oneself to give up trying - a dangerous avenue to intentionally turn down. Your belief must be truly sincere. And often that isn’t enough, the medical world can still prevent any hope in stilling the waters, quite easily.