Two Extremes of Metta

Repeating the words many times over might help you remember to do Metta when it is needed in a difficult situation. As a form of Metta it is very shallow, but nevertheless maybe beneficial. If you develop aversion when doing this best not do it at such times. When you get fed up just stop. Also the repetition may serve the basis for a change in attitude.

The most deeped way to do it is do Vipassana or Samatha Meditation. At some point you might get a pleasant feeling. When this happens bring ups the wish that others also experience this peace. This is more deeper form of Metta.

Objective of Metta is to:

1. develop and increase qualities Metta
2. break barriers thereby you consider some people of class of people more favorably than the others

What you are doing is good – keep up with the practice of Metta. The practice of Metta can be great when your character buildup is based on all four of these: däna (giving), seela (moral conduct), bhavana (mostly loving kindness towards others), and culminating in panna (wisdom). While these ones are being cultivated, viriya (effort), khanti (patience), adhittana (determination), metta (loving kindness), nekkhamma (renunciation), and upekkha (equanimity) too gets cultivated.

Here the giving (däna) is not only giving to the needy, to animals, to elders and yogis, etc. It also includes the abhaya däna. Here one knows that all sentient beings value one’s life the most, and thus does everything possible to save lives. Abhaya means remove “bhaya” or fright. Thus in doing this, Metta grows simultaneously too. Most categories are inter-related, and grow together. The highest giving is dhamma däna. It starts with teaching morals to others, and living an exemplary life.

Metta Bhavana is best for you. It is just that you do not know what a powerful tool this is. Also you seem to not know that there is a higher level to even the Metta Bhavana. Why this is a powerful tool is that it is possible to remove many of one’s bad kamma seeds through Metta Bhavana.

When a kamma (abhisankhara) is committed, the kammic potential of that kamma is deposited as a kamma seed. A kamma seed has the potential to germinate or come to fruition if suitable conditions appear. By “transferring merits” when one does good deeds, and also by doing the metta bhavana, specially the Ariya metta bhavana, one can get rid of that kamma seed by paying off that debt.

We have become indebted to innumerable beings in previous rebirths. In the “Metta Sutta” the Buddha has explained how much of this debt can be paid off by doing the metta bhavana and also by transferring merits to “all beings” when we do a good deed.

The metta that you do takes a higher significance if you feel for the beings in the lowest four realms (apayas) who undergo unimaginable suffering, both physical and mental. You mentioned about the suffering of this human world in your OP. The human world is included in the lowest five realms where beings have physical bodies that are subject to sicknesses, body aches, and getting old before dying.

Even in the sixth through eleventh realms are that of the devas, they could be subjected to repulsive touch, distasteful/unpleasant tastes, smells, and sounds, and visuals. No living being is free of future suffering. Unless the Sotapanna (stream entrant) stage of Nibbana is attained, even beings in the highest realm can end up in the lowest four realms in future rebirths.

Thus it should be your wish that they attain at least the Sotapanna (stream entrant) stage of Nibbana by comprehending the Three Characteristics of anicca, dukkha, anatta. Then one becomes free from the four lower realms forever.

When the next stage of Nibbana (Sakadagami stage) is attained, one becomes free of births in the lower five realms where suffering due to physical ailments and diseases are possible. At the Anagami stage, one removes more akusala citta and will never be born again in kama loka including the deva realm. So in this higher metta bhavana, the words have deeper meanings. When you say “be healthy” now it is meant to be healthy forever. Unlike the one that you used to do, in this one all four Brahma vihara (metta, karuna, mudita, upekkha) are cultivated.

“May myself and all living beings attain the Sotapanna stage and be free from suffering in the lower four realms forever”
“May myself and all living beings attain the Sakadagami stage and be healthy forever”.
“May myself and all living beings attain the Anagami stage and be content (attain peaceful happiness) forever”.
“May myself and all living beings attain the Arahant stage and be free from all suffering and attain the full Nibbanic bliss”.

Consider hate. Sometimes we find thoughts of hate displeasing and that turns us away from it or we find them harmful and we repress them. But having frequent thoughts of hate (say, as an escape for some pain), our mind develops into hatred. When we feel pleasure with it, we obsess and our thoughts and imaginations are colored by hate -- we imagine situation where we act on hate and we find pleasure in them. This further inspires us to act on hate. Later on, some event may happen and we act instinctively and naturally on hate because of the mind being developed on it. As this is now ingrained in ourselves, part of our thoughts and act, we essentially became a hateful person.

Then, this is the way that there is development in hate.

It's the same with metta or compassion. The feeling of being moved, even when the circumstances are sad as in the case of compassion, is a sublime feeling (probably, hence the name of brahma-vihara), and also pleasant.

As it turns out, the mind naturally inclines towards that pleasant feeling -- even though some of us are a bit scared by it and instinctively repress it. When that feeling is nurtured, our thoughts and imaginations are colored by it. So we might see or imagine a person being very compassionate or kind and think "I wish I was like that person". In this way, we become inspired. Then, our thoughts and imaginations further makes us adapt and feel comfortable with acting in the same way, as these thoughts and imaginations are pleasing.

Later on, some event may happen and, because of that development, we act instinctively and naturally on compassion or kindness. As this is now ingrained in ourselves, part of our thoughts and act, we essentially became a compassionate or kind person.

Then, this is the way that there is development in kindness or compassion.

“Bhikkhus, whatever a bhikkhu frequently thinks and ponders upon, that will become the inclination of his mind. [...] If he frequently thinks and ponders upon thoughts of non-ill will ... non-cruelty and he has abandoned the thought of ill will ... cruelty to cultivate the thought of non-ill will ... non-cruelty, then his mind inclines to thoughts of non-ill will ... non-cruelty.

-- MN 19

Finally, there are ways of developing these factors deeper through specific types of meditations, much of which have been discussed here previously. There seems to be (at the moment of writing this) 28 questions with metta in their title and a few more with "kindness" in the title. They might give some pointers to learn more about this practice.