It feels like if I try to loosen my fingers I loose some back tension.
That's the problem: “Trying” results in creeping (loss of back tension). Releasing the arrow must be an unconscious process. The release can either be triggered subconsciously, by simply holding or increasing back tension until the shot goes off by itself, or by using an external trigger, like a clicker. If using an external trigger, the trigger must result in the shot being released unconsciously, in other words, the trigger must actually trigger the release itself and not trigger the thought to release. There must be no conscious thinking involved. Why? You can't think fast enough to trigger a clean release. It's like accidentally dropping a glass and catching it midair Before it hits the floor; you can't think about catching it because by the time you do it will already be broken.
What your unconscious mind does for you is to tell your fingers to relax, and then the string brushes them out of the way. It is not telling your fingers to open, because they can't open fast enough to get out of the way of the string. That is the point of the paint can exercise suggested by Pine, because you can't “open” your fingers fast enough to get a clean drop of the paint can; you can only get a clean drop if you relax them.
If you have too deep a hook, it is difficult to get a clean release. If your hook is between the finger joints, try putting the string in the top finger joint instead. If you have a strong grip, this shouldn't be any problem for you.