There are many causes of erection problems. From mental health, to nutrition and your own or your partner's behavior. Try to figure out for yourself what it might be for you and see what tips might work for you.

• Good foreplay: Take time for foreplay, this is very
  enjoyable for both partners. For example, choose to indulge your partner with your hands
  and/or mouth. Letting your partner enjoy yourself is also very arousing for you and
  takes the pressure off of having to be stiff.
• Healthy lifestyle: No stress, plenty of exercise and eating healthy
  are very important for a healthy libido including erection. Bad for
  your erection include smoking and eating a lot of sugar.
• Foods for a better erection: Bananas, nuts, salmon, coffee, red
  wine, garlic and red peppers. These foods have a positive
  influence on blood flow and testosterone levels. Everything in moderation, of course,
  especially the coffee and wine.
• Extra stimulation: There are ways to add extra stimulation to the penis or brain
  such as vibrating toys, cock rings and erotic movies.
  These are resources that can be used with the partner. Note that if you watch a lot
  and/or hard porn, you may become desensitized to "regular" sex and
  become less quickly and well aroused as a result.
• Poses: The best positions for maintaining erection during sex
  are the 'missionary position' and 'doggy style'. In these, blood flow to
  your penis is better than, for example, a position where the woman is on top of you.
• Practicing during erection: Practicing to maintain an erection can be done
  by masturbating and stopping for a moment each time, letting the penis become flaccid
  and starting again. Focus on the excited feeling. You will learn to
  gain control over getting and maintaining an erection this way.
• Choosing the right condom: If you lose your erection during sex
  with a condom because it is too loose, too tight, or doesn't make you feel enough
  , try a different size and/or a different brand/type. Many studies
  have already shown that with a properly fitting condom, the feeling is almost not
  different than without one. Read more about how to determine your condom size here.
• Be honest about what you like: If your partner does something
  you don't like or that makes you feel uncomfortable, it can cause
  you to lose your erection. Conversely, if you make it very clear to your partner
  what you like, they will be much better able to
  maintain your erection.
• Practicing putting on a condom: If you lose your erection when putting on a condom
  it is a good idea to practice it. The faster and easier
  you get the condom on, the less it will distract you.