The only really relevant thing regarding belief is that if you're not getting the results you want, you can simply "believe" that you're not doing it right and change the methodology or learn more about it.
EFT tends to get the best results when there's a CURRENTLY experienced emotional difficulty. It's not enough just to think about something that's happening. EFT is like a weed trimmer, but you have to pull the weed up first in order to cut it.
In this sense, the "weed" is the emotional intensity of any kind.
And remember, too, the BIG deal about the APEX problem - when a problem is gone, you tend to forget it even existed in the first place, so this could be happening to.
Either way, a fun experiment, depending upon how adventurous you're feeling, might be to tap on all of your emotions about how "EFT sucks" and "I can't believe I wasted all my time on this nonsense" and the anger at "I'm not getting the results I want, damn it!" and everything else that has some kind of emotional value in your original post.
A funny way of looking at the major relevancy of it working only on NOW emotions is to think of it like bug spray, with the bug being the emotion. If you're not spraying (aiming) at a bug, of course you're going to spray with no results - nothing is "wrong" so to speak.
So try it with any sort of emotional difficulty you could think up in this moment - give it a number from 0 (totally gone) to 10 (totally intense) and see if it shifts downward. If it shifts UPWARD, then you're tapping (literal pun intended) into other areas that want to be released, too, so just EFT those.
And if you get no results whatsoever, then EFT may not be for you or you might have nothing left to clear.