I can't clear the Forrest from the shadows because my stamina isn't enough and I can't use magic anymore. Just used it once and now I can't touch the button anymore.
Bob-of-Rovia's given you the solution, above, to the magic no longer working. When your Curse level hits 100%, magic no longer works. You need to drink the tea he described to cure yourself.
Go to the kitchen stove to make it.
With regards to your stamina not being enough, just raise your max stamina by training in the garden with Robin.
Also from the skill book in the menu, Sneaking will mean that you can travel more turns without raising enemy awareness, so can travel further without being attacked, meaning less stamina consumed to reach the same distance.
Plus training the Attack skill to max, level 5, will reduce the stamina cost of heavy attacks by 50%.
Best way is just to train in the garden & directly raise stamina, though.