Hi, I have been practicing mindfulness meditation for about 37 years. The idea of mediation isn't exactly to make one feel good all the time, it's to help you practice how to cope better when you don't feel good.
Practicing meditation is kind of like saving money in the bank...the payoff is there when you need it.
I used to use a mantra - have used several - years ago, but I don't really use a a mantra anymore, I just focus on my breathing, and on just noticing the thoughts floating through my mind.
A mantra can be any word you find useful...or you can practice without a mantra.
Meditation is not easy, it takes work, practice, patience, and dedication. Tremendous commitment.