Two Extremes of Metta

I've practiced metta, or attempted to, with mixed results. Sometimes I'm wondering even *how* to practice metta, and in my mind it feels like I'm just repeating "How can I develop compassion? How can I develop compassion?" It's almost a lack of contact with the feeling of compassion, or just tedious repetition. However, sometimes, at other times, I feel extreme feeling of compassion, to the point where I clench my jaw to avoid the feeling (since seemingly I am very repressed). It's a mixture of being moved, and compassion, since it arises when I react to either moving events, or when I see someone suffering. But, admittedly, it's really being moved that generates the most feeling. I have sometimes managed to generate such a feeling in meditation by visualizing people doing acts of kindness, myself included. However, whenever this happens, I feel that the feeling isn't enough, that compassion *should* require effort. Is this the case? Does a feeling of being moved, or a compassionate intention and feeling, generate something positive? Or, am I right in thinking that without effort and discipline, these feelings are merely oceanic and vast but not linked with true mental development? Also, I suspect I might lack self-compassion or something because I don't allow myself to feel these feelings, and I often--used to--feel being undeserving of all these things. Any thoughts? Thanks PS: I recall a Buddhist teacher reacting to my yearning to develop compassion with enthusiasm, but advising against body meditation because it could be detrimental. How could I know if metta meditation is best for me? Any signs?