Hi
In the common form of weak erection, a man can't reach sustain strong erection during sexual intercourse — but can maintain the same with oral or manual stimulation of the penis. Some men can maintain strong erection only when masturbating.
Masturbation results in rough stimulation of penis which is not the same while having sex. I don't see anything other than psychological reason as the cause with this.
I would suggest you to communicate openly and honestly with your partner about your condition. Treatment is often more successful if couples work together as a team.
Generally, you can have strong erection by self-masturbation and maybe through love play, but not through intercourse.
So I will encourage you and your partner to gradually proceed to a situation where she can perform
oral sex or masturbate you, and eventually go on from there to a more 'advanced' situation where you are able to cope with erection inside her.
Another effective therapy:
first, the you can go through a period of treatment in which actual sex is off limits, but kissing, cuddling and manual stimulation is encouraged. Then progress to a stage where your partner, masturbates you till you reach an orgasm, finally, move to a situation where she lowers herself onto you shortly before you climax.
I believe this should help solve your problem, but if the problem persists I would suggest you along with your partner to consult your physician and a counselor(
psychologist or sex-therapist).
Psychotherapy can help by addressing underlying mental health problems leading to weak erection, such as stress,
depression or
anxiety. It's also used to address psychological issues that directly affect your ability to maintain erection. Depending on the underlying cause, you may benefit most from seeing a sex therapist — a mental health counselor who specializes in
talk therapy for sexual problems. The type of counseling that's best for you will depend on your particular concerns