So your fantasy is to cum and then play and cum again but you have an aversion to your cum! Don’t would seem that’s why you stop and or loose interest
If that’s the case you need to work on that first before anything else can. It’s a case of desensitising you from what holds you back.
After that I see two easy options the first being, do you have to cum the first time? A dry orgasm and your kegal muscles would allow you the second chance to cum. After her first orgasm a little play where some of your precum is there and then go again.
The second is just practice to cum twice (easy I know 
) or as you are cumming the first time use your kegal muscles to stop the flow and to ruin the orgasm so can go a second time.
More extreme ideas are cum filled ice cubes that melt while you are together the first time and then you can go for the second time.
This is an interesting challenge I’m off to search the other topics for cumming twice. I’ll add links if I find something.