How to stay ually ?

What thoughts should go threw my heead? soo ii can be able to maintain and . im 17 back then it was really easy to get super hard for a girl , as i got older it got different . i kiss and touch on her and it gets me hard , buh when im just about to put on a condom thats when everything gets baad , my goes down ... idk and sometimes its because i think about the 11 inch she took.. , well she didnt take it all , but enugh of it i guess.. and sometimes ii just think about losing itt. and it goes away . I want to banng my girlfriend okaay , and really baad , i just cant help these thoughts . they always come back to me when were "getting there" .. i just want to be able to keep an the whole time , buh its like out of knowhere i become less because of these thoughts . and i talked about it with her and it broke her heart , because we really love eachother and she thinks im not ually attracted to her , the thing about it is i am . just shes way much more experienced than me , and it causes me to think of how other guys performed , and how much bigger they were than me . i really need some advice.