The first thing you have to do is to stand up straight, with your shoulders back, and your head held up high. You will immediately feel better, more capable. Then, watch yourself in the mirror, and smile a real smile. You will be amazed at the fact that smiling makes you feel better just by doing it.
It's very hard to lose someone, for a few months, years, or forever, but we have to move forward with our own lives. I suggest that you start something new in your life, a new hobby or a new exercise regimen. This will give you something new to think about, to make plans for, and will take your mind off of the person you are missing.
Wake up each morning and try out that smile. Be grateful that you are alive today. Eat three healthy meals each day, with lots of fresh fruit. Then do what is in front of you to do. Don't worry about tomorrow, and don't dwell on the past. Live today just for itself, then, when tomorrow comes, deal with it the same way.