I don't have much experience with photoshop but for some reason it won't let me brush only within my selection (inside the marching ants). For some reason the brush crosses over into the area that is not selected. I included a video showing the problem. Any help would be greatly appreciated!
Did you created your selection using channel, like Ctrl + click on channel? Try creating selection using Lasso tool, for example, or Color Range.
You are not painting full white color but rather gray or white with transparency, right? When using channel to click and select many pixels will be semi-selected or somewhat selected, you can paint on them but not full black or white, only percentage of it based on grey value on channel from which selection is created.
It looks like you may have luminosity selection active. Photoshop only shows marching ants around the pixels that are more than 50% selected. All pixels may be select at some percentage. Save the Active selection as an Alpha channel and look at the Alpha channel thumbnail the get a better view of the selected pixels.
You normally adjust luminosity selections not use them as painting guides. To work smoothly on shadows and highlights