If the trigger is held back whilst cocking it is as if you were to ask a man to sit down and pull the chair from under him. He falls just like the hammer.
Almost all modern pistols with visible hammers have rebounding locks so that after the hammer falls, on the trigger being pressed, and explodes the cartridge, then it jumps back to half-cock of itself. This saves time as otherwise the hammer resting on the exploded cartridge would have to be raised by the thumb to half-cock before the exploded cartridge could be extracted and a fresh one put in.
Now, practise till you are perfect, using an empty cartridge.
Open, insert cartridge, close, put to full-cock, lower to half-cock, extract cartridge, close pistol.
Do not be satisfied till you can do all this without a hitch or hesitation and without letting the hammer slip.
When you do this perfectly you can go on to the next lesson, but not before.
When you have the pistol at full-cock, it can be fired by pressing the trigger, but we have not come to that yet. We are only learning how to safely handle a pistol.