“Getting ourselves happy with God”—or, to make the same point in mantrayanic terms, “taking delight in the Name”—is certainly an important key; as Schuon says in an unpublished Text to which I called your attention some time ago, one must not only negate a given passion but also affirm its positive analogue. This, however, is where you seem to be faltering and why, I suggest, you sometimes doubt whether help and happiness are to be found in methodic prayer.
Could it be that you are looking for, or expecting, the wrong sort of happiness—that your sense of what happiness should “feel” like is too subjective and worldly? I think it was you who once told me about the Zen master’s reply to the question of how it “felt” to be enlightened: “Miserable as ever!” I’m sure this is cold comfort just now. Nonetheless it’s true: the joy of a right relation with God is not reducible to a merely contented frame of mind.
You know this in theory, of course. But I suspect it remains for you something rather abstract. You have your memories of certain times when the Prayer came easily, when Invocation was indeed your life and your joy. But I suspect these memories are purely “in your head”, and not concretely grounded in your breath, your gate, your posture, and the regions of your body. Such a grounding is crucial if you are to have any hope of retrieving the “memories” in times of struggle and stress.
A purely nostalgic desire that things might be as they were at Time X will never be enough to overcome a craving for physical pleasure at Time Y. On the contrary, the only way to put the craving aside is by counteracting it, at Time Y, with something equally “physical”, if I might put it this way. But this in turn requires a persistent, consistent, practice of Invocation, such that the Name becomes rooted in our very bones and blood. This is doubtless why the Hesychasts so often insist that the monk remain within the “cell” of his body.
A final point: you lament that your invocatory practice sometimes seems to require of you a “greater strength than [you] can muster”. But in point of fact, the Name is precisely Christ’s “easy yoke” (Matthew 11:30). We can of course make the practice difficult by approaching it as “our” work, and our potential “achievement”. But in truth it should be no harder than breathing, since the Act is really God’s.