I've typically found there are two reasons that ruin my motivation for photography: lack of confidence, and monotony.
If I shoot the same model too much, or the same style, or process my photos the same way, I find that I get bored with my work. On the flip side, when I'm not feeling confident in my work, I also find excuses not to go out and take photos.
I would identify if a head or heart issue - head in that there are technical issues, or a heart issue in that you're just not interested in what you're shooting or how, and need to turn to new subjects.
As others have already suggested, I'd recommend seeking out different subjects or trying a different genre of photography altogether, I think it'll do wonders.
"I'm not feeling confident in my work, I also find excuses not to go out and take photos."
As I improve and raise the bar on what I will accept this one is a killer, the risk of coming home empty handed or disappointed starts creeping into the back of my mind. If I think the conditions are not going to be just right I won't even bother which is completely the wrong the attitude.
I can absolutely relate to this, and mostly for me it's a time thing. If I can't be outside during the golden hours, I don't even want to go out, or if it's a completely overcast day, I lose interest as well. Because I'm focusing more now on portraiture versus nature photography, I've been able to force myself out there more, but my ego sure does put up a good fight.
I've found that the quickest inspiration for me is to find work that I immediately think "Damn...I wish I'd taken that." It gets me motivated to get out there and keep working. I'll spend hours pinning, ripping photos out of magazines, or pouring through books at Barnes and Noble.
You can also find some AMAZING photography documentaries on iTunes and Netflix. One of my faves in particular is on Annie Leibovitz..."Life Through A Lens". REALLY great stuff. Super inspirational.