From my own personal experience the shift happens when you discover the core belief, rather than the core memory. The first time it happened for me I knew that I'd found it because all my physical symptoms stopped and I felt really different.
My recommendation is not to get too caught up on finding the "core" of the issue. The most important thing is to tap on whatever is presenting itself right now and follow the trail of what comes up for you. You cannot force yourself to get to the core of an issue - you often have to peel off the outer layers of an issue before the core of it reveals itself. This can take some persistence!
Something else to bear in mind is whether or not you are accessing the feelings involved sufficiently while tapping on your own. I got nowhere with the core stuff working on my own, I just got so far and didn't really know if I'd shifted enough. When I sat and tapped with my partner who was also learning EFT, all parts of me seemed to feel safe enough to access the feelings and the beliefs and the big shifts started happening. If you are only tapping on what you were already consciously aware of when you started tapping, and no other feelings or beliefs are surfacing, it is likely that you might need some support to engage your subconscious.
One more thought - you are unlikely to "figure out" the core issue with your conscious mind. If your conscious mind knew what the core issue was, it would have been able to resolve it. The key is to tap and befriend your subconscious mind/inner child - that's the part of you that knows what the core is and it will present it to your conscious mind to work on when it feels safe enough to do so.
Best wishes,
Jo
Detailed Book "You CAN Heal with EFT" available FREE from:
www.selfheal4me.com