Once you hit that slump and aren't motivated anymore, step back and take a breath. Look around you and see what the friggan heck is going on. For me, I noticed my sleep is terrible. I haven't had a good nights sleep since Olivia was born. I'm constantly woken up either in the middle of the night or really early in the morning.
So what I do is I force myself by placing the camera on the tripod and setting it on a location where I'll shoot. Once I start clicking and rolling, I guess that's when I start working. I also stop myself if I see that I'm not having fun because the worst thing to do is make the thing I enjoy, an annoyance. I started it sure, but my heart and mind isn't there. So I take a day off and try again the next day. Sometimes motivated and sometimes not. Depends on the sleep I get.
I'm 35 years old as of this writing. I'll be 36 in 4 months. I still remember High School days like it was yesterday.