Hello, everyone!
Today I'm 365 days sober. But I've noticed recently that it's getting increasingly harder for me to stay sober. I'm about to finish the 9th step in SA, I'm working with a therapist but I don't seem to have the same motivation as before when I just started. Back then, the program seemed fresh and interesting, I had a girlfriend on who I didn't want to cheat but we split up some time ago, now I'm a single young man and it's hard for me to answer the question "Why not". I'm not very religious so maybe this is the wrong place to post this question but I'd appreciate your feedback anyway. How do you motivate yourself to stay sober. Thank you.

The motivation has to come from yourself for yourself.

Think about how low you feel after a fall.
Think about the wasted hours looking at porn, and what you could have achieved for yourself and for others instead.

And I hope you can find a wife.

While it's good and helpful to have external situations which motivate you, at the end of the day the motivation has to come from you. You have to get yourself motivated because of yourself, not because of a wife/girlfriend, etc. 
For me, the motivation is that I am a Jew and therefore i am supposed to hold myself to a higher standard. I think about how terrible I'll feel if I fall, and how disappointed G-d would be with me. Like ColinColin said, a good motivator is to realize how much time is wasted with this inappropriate stuff, and how destructive it is to the brain. The mind is a powerful thing, and i find that when time is wasted on this stupidity it cloids my mind and makes me less smart. It's easier to think and brainstorm when you have clean mind. 
Have you spoken to your therapist about this? They might be ale to guide you better as to what your motivation should be since they know you better.
If you want to talk more privately, feel free to email me 360gye@gmail.com