PD09: If every pleasure were condensed and were present at the same time and in the whole of one's nature or its primary parts, then the pleasures would never differ from one another.
Pleasure can be examined in terms of intensity, location and duration. If you really look at your present experience at any time, you may find that you're happy even though you just stubbed your toe. Or that your toe feels intense pain, but your belly is pleasantly full. As to mental pleasures of the bittersweet variety, I think of them as comparable to multitasking. Current neuroscience (to my understanding) has found that multitasking is in actuality just rapid task switching. In the same way, I would posit that bittersweet is actually bitterthensweetthenbitterthensweetetcetc. The pleasant memory prompts the pain of loss, which might then be replaced by a pleasant memory and so on. Or a pleasant memory may prompt the pain of loss, and the pain of loss lingers. Or vice versa.
An experiment that I occasionally do is when I feel like I'm in a neutral state, I try to really examine how I'm feeling. I always find that I'm experiencing pleasure and/or pain: it's just that the intensity may be very low, or a pleasure somewhere is offsetting a pain elsewhere. We are constantly experiencing pleasure/pain, both as a complete organism and in our various parts. Some of these concepts need to be felt as well as reasoned out, which is part of the point of the Epicurean canonic.