I feel that 'happy' is a term that describes a temporary, fleeting emotional state. It's associated with feelings of joy, elation, etc - but our experience with those types of emotions always tells us that it's always a transient state and never lasts. I mean no one feels and experienced joy and elation 24/7, it's just not practical or realistic.

So maybe rather than telling yourself that you deserve to be 'happy', try telling yourself that you deserve to be at peace, to feel peaceful, to feel calm and content. You deserve to find your peace of mind. I mean who would complain about experiencing a state where they predominantly experience the absence of any 'negative', debilitating emotions? I think most people would be content with that. : )

It is true that internal peace/contentment needs to come from within. Here is something to contemplate. Rather thank approaching this by thinking that you need to add some specific collection of beliefs to your mindset , or that you need to manipulate and control your external life circumstances (impractical) - try to adopt the conceptualization of removing beliefs, thought patterns, and conditioning that is not helping you and not contributing to your well-being. Really scrutinize your compulsive ways of thinking and the thoughts you harbor, the beliefs you hold about yourself, your self image, your identity - and sit there and ask yourself if they are helping you feel inner peace, contentment, and acceptance - or causing turmoil, tension, and stress/anxiety. When you identify what's not working for you, you need to push yourself to let these things go, to drawn conscious energy away from identifying with these old beliefs, thought-patterns, and prior conditioning that is not serving your best interests. It's almost like you arrive at the point where you say to yourself, "Hey, this isn't working for me anymore, I'm ready to try something else". And then you gradually integrate a new way of perceiving as you distance yourself and eventually abandon your old way of looking at things. Be patient with yourself, this takes time. Don't expect overnight success. Imagine yourself taking small steps, and over time those small steps will add up to significant gains/progress.

So again, don't strive for 'happy' (joy, elation) and leave yourself feeling disappointed when you cannot maintain that temporary feeling - instead strive for internal peace, balance, contentment, calm, acceptance. Those qualties can be internalized and sustained (maintained) over the long haul. Sure there may be some disruptions along the way as life naturally unfolds, but you will find yourself reverting back to that state once you have come to find it and identify with it.