After only getting off by hand for most of time,both dicks nerves and brain get used to that sensation so basically its the way your body gets satisfied. So when it comes to pussy,which is completely different from hand by both warmth,moisture,tightness your nerve gets confused. He is like "well this doesnt feel like rough,dry hand.Its not what I used to ejaculate so I will stop erection."
Basically its not sensitivity brain got used to,so you need to rewire to real deal. Honestly pussy is divine,and its really shameful to waste dick on hand when you can have something much better. Pussy gives both physical AND emotional pleasure.
All in all just stop jerking and your brain will slowly but surely loosen wiring process to hand. Good luck mate!