How to deal with stress

I don't why I always stress so much. Everytime I'm about to have sex it's like I'm about take some real hard test or its something live or die . I'm told most of the time that I'm excellent and too good but I just don't know why it still happens . Recently this thing is bothering me and gives a hard time to start of however once it gets to it it works fine . Please help me to know how I can deal with. I just have zero confidence on myself .