Most of the people don't notice it, but we do wake up after each REM is over. Most of the time we just turn to the other side and keep sleeping, and that's fine for ordinary folks, that don't lucid dream. It's not any "big awakening", just still half asleep. And that point is when you should condition yourself to start noticing. Perhaps just by thinking about this often will help you notice it.

You say mantras don't work. Have you tried one, that wakes you up at certain time? We have this ability to wake up at certain time, like when you travel, or have to get up at not normal time (at least, it works for me).

So maybe you can set up this mantra to wake up at later time in the night, when most of the sleeping time is in REM and you are likely to be in a dream.

Another thing is to drink water before bed - it should wake you up at night and you may get lucky. Happy dreams