The reason you’re able to drop a load of cum on your girlfriends face only at certain times is probably because that is the time when your semen levels are full. The body takes a complete day to produce semen, so when you masturbate or have sex every alternate day of twice a week, you can enjoy squirting loads of cum, but when you masturbate or have sex regularly your semen levels would be normal and hence you will be able to merely drop your juices on her face.
No, you’re not bad or is there a problem with you. The reason you’re unable to spurt layers of cum is mentioned above. Try it out right away. Abstain from masturbation as well as sex till your girlfriends birthday and that day, you can give her the massive facial she has wanting to get. Good luck.
Squirting doesn’t have anything to do with fitness. Not all women are capable of squirting and it has nothing to do with the body structure/physique. There are fat women who can squirt, so your assumption is wrong. And btw, I'm pleased that you take precautions while having sex, its good to know that one is aware of the basics of sex ed.