Sounds like you're either too preoccupied with making her get off. There's a point in the game where you simply have to take a selfish moment to "fire your gun". Like you say, you probably are sooooo into the duration of the touching and rest of it, that to you the longevity, never wanting the endorphins of the feel good session to end. Let's face it, after anyone has ejaculated once, that first of all takes a lot out of you, it's the male version of a climax and for the most part, anything after that will be pleasurably less intense. Which means the penis will eventually go back to a more or totally flaccid state. It can even be painful if she keeps the pace up immediately after ejaculating. There are some women I've been with that are similar, the session could last forever, then there are those that I even wished they'd disengage and leave.