Your body has not been designed to suck your own dick bit some men with big cocks are able to perform this feat. You could work on stretching and making your body more supple and you could try and stretch your cock slightly by daily hard wanking. It sounds like you have an average size cock so do not threat about it. The Chinese and Japanese are experts at sucking their own cocks so you could ask at your local Chinese restaurant for advice

My dad used to suck his own dick and I grew up watching him, he used to go down to his shed in the summer months and wank over porn mags and then when he was about to unload he would duck his own dick and swallow his load he didn’t know I watched him but he used to do yoga and stretches daily so he could suck himself. He also had a huge nine and half inch cock

Why would you do that?