I Ejaculate immediately when I get an erection

I've made a post about this before but didn't get any solid answers. I am a 17 year old male, and I cum immediately every time I get an erection. I'can be sitting on the train in public for example , get a random boner and once I'm 100% hard I cum right away. Ive tried to stop it but it's almost involentry. And yes, it feel really good so it's not like this is happening without orgasm. I can't masterbate or have sex with girls because of this problem. I've tried jacking off when I'm flaccid but I get hard very quick and the same thing happens. This is happening at least twice a day and I just don't know what's wrong with me. If anyone could give me some kind of answer that would be amazing because I'm too scared to tell my parents.