Merlot versus Cabernet Sauvignon?

[quijote]

One of the roadblocks I've encountered in my quest to develop an ability to distinguish aromas and flavors of different wines is that, in many cases, merlot and cab seem very similar to me. At the beginning of my explorations, much of this had to do with the fact that the two are typically blended in Bordeaux and some New World wines, so the untrained palate cannot readily pick up on the differences.

The tastings in Andrea Immer's book have helped a bit, but the differences seem only to apply to the wines that she has listed in her book. Some unlisted merlots I've tried have been very full-bodied like cabs, and some cabs I've tried have been very medium-bodied like merlots. Also, I've found that, at times, the flavors and aromas ascribed to one can be found in the other.

Is this just me and my inability to tell a merlot from a cab? Or do other people sometimes come up against a similar situation? It's very frustrating.