Feeling down today

I have nothing to be sad about- but I am feeling a lingering depression today.  I am still drinking, why I don't know.  I really have to just not buy beer or if I drink just in social situations.  I have such good intentions but I always make excuses or give in mentally.  Like tonight, I tell myself I will not buy alcohol.  I will do some errands, go home and then go to the gym.  I am so sick of feeling so frigging dependent.  Why do I have no willpower.  I really want to control this.