Where does it say or who said that one shouldn’t daven if it’s meaningless to them or if they are not doing it with their full heart?

No, that's not the Jewish approach, for it's better to at least pray, even without kavana (concentration), rather than not to pray at all, for the very prayer itself is a declaration of intent. "Wanting to want" is also a certain minimal level of want. What you probably heard is the mishna in Brachot 4, 4, that is basically translated: "R. Eliezer says: If a man makes his prayers a fixed task (keva), it is not a [genuine] supplication.” As with anything in life, one should bring himself to just "bear down" and concentrate to the best of his ability.