If you want to feel good, then… feel good!
It may be near impossible to follow such an injunction, it’s still a sound idea. We can’t experience two different emotional states at the same time, and though most people can’t change their feelings at will, we can all choose how we think and act.
Taking steps to be compassionate towards others, for example, or practicing altruism by putting their needs before our own, enables us to feel more generous and considerate, which in turn helps us to feel good.
If you can’t manage such noble notions as altruism and compassion, start with something simpler like caring or showing interest in others. The rest will follow.