You probably heard all this before / will ignore this but nonetheless this is how muh mummy taught me;
Get a sharp, serrated bread knife, hold it in your main hand. On a cutting board, hold the bread firmly in place with your other hand, and put the knife on the bread at the thickness of slice you want, making sure it is parallel to the board and to the side of the bread. To slice, rapidly move the knife back and forth while also pushing downwards, in a sawing effect (making sure to stay parallel). make sure you hear the knife hit the board before you stop, and make sure to use your whole arm in the sawing movement (if you just use your wrist etc it will come out wonky)
For you, you may wanna try some breathing exercises first to focus on steadying your hand