Have always heard that fiberglass is essentially bendy stuff, while epoxy is stiffer and more brittle. If you epoxy the core to the fiberglass, eventually the fiberglass is going to bend, and the stiff and brittle epoxy is going to crack--- putting you just about back to where you are now. We replaced sections of our delaminated and rotted out cabin top core with polyester resin, NOT epoxy, for this reason. With the same materials throughout, the whole thing bends together (when and if it does) and holds together.
Stick with it-- it will certainly stick to you!