hi bro, i was having similar issues a few years back (i'm older than u!), i could get rock hard for 10+ minute stretches, but cannot maintain hardness all the way for marathon sessions like during my younger days
did a bit of research, tried a few things and it helped, sharing my tips here for u and other interested bros:
hardness is mainly about blood flow, how to improve blood flow?
1. cardio exercise to keep the heart pumping strongly
2. take care of health (check for silent killers like diabetes and heart disease)
3. reduce/avoid/minimise stress
do also note some medications affect blood flow:
a. blood pressure medication
b. allergy medication (antihistamines)
c. nasal decongestant (specifically pseudoephedrine)
d. antidepressants
tying in with point 2. take care of health to minimise usage of medications, or try to avoid taking these meds before sex
hope this helps!
