The note idea is not bad (but it obviously depends on how you feel about it). However, if you do end up talking just remember that the message that you are trying to send is simple...
CA, if you really think about it, all you are saying is.
Is it a good time to talk > you've notice different behavior > there is a reason for it > I'm a procrastinator and view explicit material and act out > I'm ashamed of it and I'm working on myself> I thought that it would be important for you to know and your support will definitely help me get through it.
I'm sure that your mom will take it well but don't worry if she doesn't. Your mom may be a bit shocked (I doubt it) and may not take it well (I doubt it again) but don't worry about it she will eventually understand you and will apologize for not being supportive (unlikely).
When I face a tough situation I always think of the worst case scenario, accept the outcome and then act. The outcome is always better (than worst case) and it is a huge relief once it's done. If you are sure that you want to talk about it, do it today! DON'T PROCRASTINATE!
Best of luck!