what shoud i do?

I have worked in current company for almost 3 years, Honestly, I love this job and work hard. Fortunately, my boss and my manager treat me well, they all helped me a lot and told me many skills about how to do things better. But now I find that I have no enthusiasm on my job. One reason is maybe I know well about this company now. Another is since I lose my last love 4 months ago, I still can’t get over it. I am about to burst into tears at lonely moment and I don’t even know why, I feel like I am wandering through life without purpose everyday. I always think that I am a loser, and want to get away from this unhappy place. So I don’t concentrate my energy on my job.My manager always tell us,the good mood is important to work and live. I also remind myself, something must change, but you know,it is easier said than done,in fact it is so hard to make some change.I really don't want live like this, What should I do?help me,God!