Someone once asked me if I was an optimist or a pessimist, and I replied that I was an optimist -- because I had read the last book of the Bible!
What I meant was this: No matter how discouraging things seem, the Bible tells us that God is ultimately in control, and some day He will intervene to destroy all evil and injustice. That will happen, the Bible says, when Jesus Christ returns to usher in His kingdom of perfect righteousness and peace. The Bible says, "But in keeping with his promise we are looking forward to a new heaven and a new earth, the home of righteousness" (2 Peter 3:13). Is your hope in Him?
Admittedly some people always tend to look on the dark side of things; it just seems to be part of their personality. (The opposite is true, of course; other people always seem to be cheerful and optimistic.) But no matter what we're like, God wants us to lift our eyes to Him, and take comfort in His promises. If you truly believe, for example, that God loves you and is always with you, won't it make a difference in your attitude?
God has not promised that we'll ever live in a perfect world -- not until Christ comes again. But He has promised to be with us always -- and you can trust His promises. Put Christ first in your life, and then ask Him to remind you every day of His constant goodness and presence.