Stop forcing yourself to be happy
Forcing yourself to be happy is dangerous and self-betrayal. You must let yourself feel the sadness, the frustration, the anger, the grief, whatever heavy emotion that comes up so you can release it.
Self-love is fully accepting yourself, unconditionally. This means also accepting when you are not feeling joyful, happy, etc… Repressing the heavy, uncomfortable emotions will hurt you in the long run, whether physically, emotionally, or mentally.
Give the emotion space to be, observe it, ask yourself what is causing you to feel this way, remind yourself that this too shall pass as everything is temporary, appreciate what it is teaching you about yourself.