My Mistress wanted, and so did I, the same thing. We tried a prolong/desensatize spray which didn't really do the trick, we learned you certainly have to wear a condom when doing it I could still feel warmth but it ended up decreasing my erection and her enjoyment.
You can look into tantric techniques but they take time and comitment, I had read that if your Mistress helps in these sessions it can help as you are involving her and so you're excitment levels during sex aren't quite as heightened.
In the end I purchased a hollow strap on from adamandeve for about $25 that had a 2" elastic waste band and suprised her with it one night. Use lube and put it on before you get too excited, if you find you can still feel something you could then use a desensatize spray. The strap on is bigger than me so it is an extra added bonus for my Mistress that she got something bigger and for aslong as she wanted! Certainly helped with her giving me a little verbal humiliation too

Good luck