Mental strength is overhyped. Dollars to doughnuts your technique is not solid. If its not solid - you will randomnly miss shots.
Reading this board I have found that lots of tennis players love the idea of improving their tennis radically via 'mental' work. But that comes in more when you are at the top of the game.
The difference between two top ten pros might really be mental strength. The difference between two rec players is one guy has a horrible forehand/serve/volley etc..
If you are double faulting your serve - drill on your serve technique. Learn a kick serve that you feel very confident in..