I've always fantasized about cumming in my own mouth and swallowing my own load but like many have written here, once I cum, I lose all desire.
After years of this fantasy, I've finally figured out how to accomplish my goal.
After a good jack off session, I shoot my load into a plastic baggie. Once done, I tip the baggie diagonally until my semen collects in one corner. I then tie that corner off with a not and put it into a glass of hot water.
Then I sit at my computer and watch some nice videos or read stories right here on Solo Touch and start jacking off again.
Usually, within 30 minutes, the desire returns. I pull the baggie of my semen out of the water and put the knotted corner in my mouth, while I continue to stroke myself.
Just before I'm ready to cum again, I suck hard on the baggie until it pops in my mouth. Once that desire returns, I really enjoy the taste and feel of my own cum as I swallow it down, usually cumming again at that point.
If you share this fantasy, give it a try. It works. The trick is to get past that loss of desire right after cumming in the baggie. Fight through that urge to just throw it away (I've done that many times). Once you have got used to it, you'll do it many times.