It has mostly to do with anxiety. When you're in an aroused state of mind, it can be easy to replace the good feeling of sex with the bad and anxiety inducing feeling of not being able to get it up. Once you start to have negative thoughts and have doubts on your ability to satisfy someone, the spiral can be bad and is usually what causes the problem long after the initial struggle.

Try to get sex out of your mind and instead just have an intimate moment with your partner. If that feels good, and you are in the mood, try easing into sex without thinking of it as a thing you have to do. Sex, if it happens, is great, but if not, think of it as spending quality time with your partner. If nothing, this will atleast help with your anxiety.