If you belive it, then it is. Nothing is out of your control to accept or deny. Only you can decide how to react. Even in moments of great stress.
When you experience stress, remember, you created the stress. Yes, contextual circumstances do influence our abilities to control feelings and behaviour; However, regardless of this fact; we feel, we act; not someone else.
Feeling Good -always
Given we have complete accountability for how we think and act it follows being constantly satisfied without exception is fully within our control.
So why are people sad sometimes.? Why are there mass murderers and suicides? It is all about attitude. The foundation that underpins your ability to manage personal behaviour is attitude.
Who cares?
It begins with caring. At the center of your being what do you care about. This is in fact the purpose of your life. It could be a God, seeking maximum pleasure or helping others or anything else.
Once you know what you care about crafting a positive attitude is easy. If even a small part of you has doubt or questions what you care about, you will find maintaining the attitude becomes more difficult.
Mix and Match
A single purpose might not resonate with you and you might mix them. For example maximum pleasure and belief in a God.
Ultimately though for this to work the core purposes cannot conflict. You must be able to percieve their meanings as working in harmony.
Over simplified
One suggestion, before you convince yourself this is an unrealistic oversimlified approach, give it a try.
You might not achieve 100% personal serenity consistently, speaking from experience though you might be surprised at how close you come.