Photoshop was my first step software when I entered to learn VFX. At the beginning It was bit confussing when it comes to use layers,I use to get a bit confuse which layer should be kept up and which layer should be kept down.and why they needed to be arranged,later I started knowing the Reason,second tough thing in Photoshop for me was use of pen tool,at starting I never succeed in using pen tool because I was unable to make the exact matching curve required.I always used lasso tool for the purpose of cropping something from a picture,but later I came to know the Importants of pen tool as their use in making Logo designs in vector format which a lasso tool cannot do,so again I stated my tough journey to understand pen tool,I did it for a whole week,took lots of tips from Google,and now I know its important and I can make any curve using A pen tool,third most tough thing is to do colour correction because Photoshop has lots of option which deals with colour correction,different ways to do it,but yes has I said there are lots of stuff which can be use for CC (colour correction) its possible to get the comfortable colour we need,