I am something of a difficult bottom--not in the cleanliness sense, but in the "mouth writes a check my ass can't cash" sense.
It's like when you're dining out and just kinda peckish, but for god knows whatever reason you decide to order three cows, a school of fish, and an amount of cake that can be measured only in parsecs; you fuckin want it bad, and at first it's pretty great, but as it goes on, there is an increasingly nagging sense of "at least i die doing what I love." Or for the difficult bottoms like me, it's the "ah ah ah...*sharp inhale* hold...hang on...just...pull out!" moment.
If I cum first when getting fucked, it's game over. I instituted a policy of not being allowed to jerk if I'm bottoming, primarily because I fucking hate it when a trick gets weird and uncomfortable and wants to stop, and I don't want to be such a trick. It's like anything else, practice, practice, practice (Norco helps, too). To wrap this all up (fucking condom pun, really?) the cautious, delicate cumdump should be realistic about taking cock; don't have to impress anyone, or be the first ho on the block to take a cock as thick as a rugby player's thigh.
There was one guy who was pretty hung...dripped precum like I couldn't believe (or stop lapping up)...he fucked, I came and clenched, he got stuck. 15 minutes later and two loads lighter, he was free.