Hey I Am Nineteen And I Can't Seem To Keep

Hey I am nineteen and I can't seem to keep an erection by myself even I'm very very worried I've had this problem for atleast six months I can get an erection but I can't keep one my heart rates down I'm not nervous I never had any issue before sometimes I have a hesitation to pee but no discharge no pain when orgasming no pain at all really I don't know if I have an std or what