Rob, you should take at least some comfort knowing that you are not alone with these feelings. Many people feel depressed with the advent of a new day. If you have specific issues that are weighing on you, it may be a good idea to seek some counseling.
If you are just referring to 'days like these' once in a while, well then, join the club - the club called life. Everyone goes through good and bad times, high energy periods and low energy stretches, as well. Just realize that there will be those times, but they will not last. Every night is followed by another day!
The keys for me are this:
1) Work on what you can do in the present moment. Don't look back unless it is for the purpose of learning from yesterday and moving forward in a more positive way.
2) Count your blessings every day. Take just a little time each morning to review all that is good in your life, from the littlest things up through the the grand gifts that are yours!
3) Find purposeful things to be involved with. If you have a job, be the best you can be at it. If you are not involved in the community, look for a group you can join. And if you have the time, find some service that you can provide to others. Even small acts of service to others can uplift your spirit. Any way you can, try to immerse yourself in something that occupies your time in a meaningful way.
4) Finally, if you have not tried it yet - develop a relationship with God, specifically Jesus. Read a little each day from the life of Christ - the new testiment of the Bible is a great place to start. Pray each day and ask God for his help. If you have not prayed before, you can read Part II on How Do You Pray.
"This is the day the Lord has made, let us rejoice and be glad in it!"