Hi
It is not uncommon to have this kind of sexual desire at this age. Sometimes it is irresistible also.
What you need to do is give specific time for work and specific time for your personal needs.
This kind of sexual desire can not be supppressed totally but can be diverted and limited to specific time.
For that, you can remind your Brain to focus on work whenever this desires interrupt your work. You can reduce contact with your girlfriend during working hours
Next Steps
By repeated practice of thought diversion therapy, you should be able to control.
Health Tips
Yoga or Meditation should help you.