The psychological causes of erection difficulties are most often related to fear of failure, performance pressure and anxiety. There may be relationship stresses which make the problem worse. You really need to see a therapist for psychological treatment.

The first stage is to understand all the things which are causing the problem. There is often more than one cause. As far as possible, it is better to be seen with your partner if you have one, as it is much easier to overcome the psychological causes with the help of a partner. This is especially so where the problem only happens in the relationship.

Once the causes are understood, the next steps will be agreed with the therapist. Sometimes there are Sexual Abuse issues to be managed. Relationship stresses may need to be resolved. Any unrealistic expectations will have to be changed.

In the end, the most effective treatment will probably be sex therapy, but at first you may not be ready for it. Your therapist will guide you through it, and deal with any obstacles along the way.
A userful book is The New Male Sexuality, by Bernie Zilebergeld