Here is some more information, hopefully, you will find of use.
This I find I use most of all in layers.
Select Layers - New - Layer
Using the mode dropdown select overlay
Then tick the Fill overlay-neutral colour box
This will give you a new layer.
Towards the bottom of the tools bar there are 2 large boxes with a small replica set to black and white.
Click on this to set the large boxes to black/white. The top box is the foreground colour and the bottom box is the background colour. To the right of these boxes is a line with two arrow heads. This flips flops the two boxes between FG and BG.
When you want to darken an area, set the FG box to black. To lighten, set the FG box to white.
Click on the brush tool and set the brush size to a size a bit smaller than the area you want to change. There will be a box on the tool bar called opacity. This controls the amount of darkening/lightening. Set this to about 25% to start off with. If you want to darken an area then set the FG to black and run the brush over the area that needs darkening. Change the brush size as required. To lighten areas, change the foreground colour to white and brush over the necessary areas.
If you mess it up, go into the window dropdown (in PS), click on the layers option. Go into the new layers window and move the layer into the dustbin.
Once you are happy with the change, click on the layers option, from the tool bar, then click on flatten image.
In curves, I click on the centre of the diagonal line to fix it. Then I go up the line to where it meets the next set of dotted lines. Where they cross, I click just above them (only a tad). Similarly, I go down the diagonal line, from the centre to where the lower dotted lines cross and click just below them. to make an 'S'. To lighten the image make an inverted 'S'. You will need to experiment but it is only a small adjustment. I hope this makes sense.
This may sound very complicated but if you follow these instructions as a play in PS, I hope it will make sense.
Please do not hesitate to contact me if you are still having problems.