Sex "problems" Bot-Top

Hi. I'm 22, gay. So , I've had sex with this guy during this weekend and it was amazing, but as soon as i tried to go as "active" , (after i started rimming his *** ) , i lost my erection and went in panic. I ve always been passive with this guy (we made it 3 times). I have not much sex experience but i've always been "bottom" and i went "top" just once in my life. Fact is that i really like bottoming, but i also want to top, i feel this desire, but looks like i also feel not being able to. Could this be just a mental thing that makes me panic becouse i feel not being able to , or is it just the fact that i will only be bottoming for my whole life?I dont like the "roles" , I like versatility and feel useless at being only bottom. what do you think about?