jd, gymrat made great comments. A man should be able to enjoy masturbation for an hour or more. To fine tone his penis. To fully learn it's pleasure. The same as a male working in the gym to chisel his body. A quickie is only causing sexual issues.
You did not state if you have a problem with a erection. For self pleasure or awakin with a hard penis. If so more discussion with your doctor is needed. Also if your having daily sex with your gf. I would limit the long jerk offs to twice a week. Always after intercoursce with the gf.
You need to be in the moment, but not total thinking of
the ejaculation.
Enjoy the pleasures with your soft penis . Cover her body and yours with chocolate and whip cream. Or draw or write with water paint. I don't know your penis size, but I am able to have some entrance with a soft penis. You both need to enjoy oral sex; it will be more comfortable for her when your soft.
Why sweat it! If you go soft changing positions. It give you a break. Just keep stoking her with your fingers or tongue. Also it good to have a sex toy or a XL cucumber / vegetable
to help her have a few more O.
Spend as much time naked together, in the bed, shower, watching TV. Enjoy foreplay all the time. Enjoy sex anywhere and everywhere. Limit the times your mating in hers or your bed. Do not take the pill for now. Ride her as you would a bike. If you fall off get back on. Let hers and your body do the talking. Put on some baby making music; and let your brains rest.
This is normal for your age. As a virgin with limited or none nude experience. Your sexual desire is stronger as a teenager and twenties. But if you get control of penis now. You should not see another decline until your 60 - 70your
Stay HEALTH, you are the the fastest Sperm from your Father.