Welcome to HCM.
Maintaining a strong erection is controlled by the brain. If your thought process gets distracted during sexual stimulus, erection will become weak. Similarly if sexual stimulus is interrupted by a feeling of anxiety
, fear etc. the organ will go limp.
First build confidence in yourself that you are going to have a strong lasting erection and do not let any other matter disturb your thought process, when engaging in sexual act, fantasize some sexual aspect which naturally stimulates you and concentrate on the sexual act itself.
Certain drugs for stimulation are available but are not recommended routinely.