I'm assuming you are new to PE. Just like going commando at first or wearing a cock ring initially, getting fully or partially erect is to be expected because of the excitement and newness of the activity. Eventually though things calm down. Regarding PE, I suggest not doing it in front of mirror or if you are feeling especially horny. Concentrate more on the exercise and doing it correctly, rather then thinking about your penis. I've been doing PE for many years, especially jelqing, and even now I occasionally almost reach the ponr because I still dig doing the exercises.