Many of us can't maintain an erection during intercourse.
For most, it's nothing physically wrong, but merely psychological.
That's right, erectile dysfunction is usually all in our heads.
We're psyching ourselves out from performing and we're letting our anxiety take over, but it's time to put an end to all that.
I've done all the research for you and here are the most likely reasons why you can't stay hard during sex.

• You're nervous - this is the most likely cause and what MANY men of ALL ages (but most frequently those who are inexperienced) have to deal with.
  Performance anxiety occurs when your body begins pumping adrenaline through your veins; this triggers the "fight or flight" response, which causes your body to restrict blood flow to the penis, so that blood can be regulated to more important muscles.
  
• A lack of sensation - you may notice that the reason you're losing your erection is because you just don't really feel any pleasure.
  This is usually tied in with being nervous, but another reason could be because your penis simply isn't used to this kind of motion.
  You're so accustomed to the pressures and strokes of your hand, that a vagina is simply too foreign to your penis.
  It's impossible for a vagina to perfectly replicate the precise feelings and motions of your hand.
  If you're a frequent masturbator, it's recommended that you tone it down and attempt to use strokes that are more equivalent to that of a vagina.
  
• Not comfortable with your partner - insecurity or a lack of comfort causes impotency in numerous men.
  In this case, the simplest fix is just time.
  It's going to take time for you and your new partner to adjust and slowly become comfortable with each other.
  

As you can see, the leading causes of psychological erectile dysfunction are nothing to be afraid of.
A lot of it mostly has to do with the comfort level of you and your partner.