This is true of any law, there isn't actually anything except common agreement that constitutions should be treated more seriously giving them extra status, and there isn't anything but the consent of (enough) people giving laws any power: nobody is ever forced to follow any law, they are simply punished if they don't.

However, once you decide to stay within the law it is indeed possible to have EU treaties stand above the constitution. In situations where you've added bits in your constitution that the EU treaty has priority, acting like this isn't true is simply breaking the law.