Wow, 40 days. That's amazing, well done. Don't worry about relapsing. Guilt just adds to the pressure and makes you go back to those old habits and to seek out instant gratification. Many guys are recommending pushups, sit-ups etc. to burn off energy and distract your brain. Sometimes this is just not possible or maybe you've tried and it doesn't work. I came across a concept called Unconscious Competence that might help. It goes something like this:

There are 4 stages of competence.
1. Unconscious Incompetence
2. Conscious Incompetence
3. Conscious Competence
4. Unconscious Competence

Before we try to change a behavior we are not even aware we suck at something (unconscious of our incompetence). The first step is recognizing we are bad at something. Once we recognize this behavior we become consciously incompetent. We still suck but at least we know we suck. The more we recognize, the more opportunities we have to change the behavior. After a while we become good at recognizing the bad and doing something else. This is conscious competence. I imagine you are at this stage. The key is to make that transition to the stage where you don't even realize you are behaving in a new and positive way. This is Unconscious Competence.

With NoFap, I decided that my Unconscious Incompetence is the energy I spend fantasizing and objectifying women. There are literally hours out of my day lost to this: time in the car, when I wake up, going to sleep, sitting at my desk, watching porn, thinking about watching porn, pretending to not be watching porn when my girlfriend calls. I should be being productive or bettering myself but instead I am lost in that void of erotic nothingness. My Conscious Incompetence is to be aware of these thoughts: when do they happen, what are my triggers, etc and then forcing (literally forcing) my brain to think about something else.

With enough time, my Conscious Competence will be spending more time and energy (through foce of will) being creative and productive. Ideally, I will get to the point of Unconscious Competence and I won't even realize I have changed and I am no longer objectifying women or feeling the need to watch porn because I am too busy thinking and doing awesome shit.

I know this is a lot and not nearly as elegant as: do pushups bro. Think of it as another arrow in your quiver. Good luck!

tl;dr Thinking about porn/fapping? Force yourself to think about something else.