Mettā doesn't have to be a warm feeling. If you think about other people’s well-being and take that into consideration as you plan your actions, that’s an awful lot right there.
"I’ve heard many people complain that they can think thoughts of goodwill [mettā], but they don’t get any warm feeling out of it. Well, it’s not necessary to have the warm feeling, as long as you think about other people’s well-being and take that into consideration as you plan your actions. That’s an awful lot right there." ~ Thanissaro Bhikkhu "Goodwill in Heart & Mind"