You're a very special creation of God's love, so don't worry about a few blemishes. Imagine how self-righteous and proud we would be if God had made us all perfect! Instead, He gave us all a few blemishes. God doesn't want us to look at the blemishes; He wants us to look at the good.
Every time you're tempted to think negatively about yourself, thank God for something He blessed you with instead--good health, a sound mind, or some quality or talent that others admire in you. There are so many things you could thank God for! Think how much worse off you could be--and think of others who are. Pray for someone who is battling a long-term illness or coping daily with a debilitating handicap. Job was delivered from his problems when he prayed for his friends (Job 42:10).
Take a positive approach and pretty soon the instigator of your negative thoughts, the old Devil himself or one of his henchmen, will give up. When your spiritual enemy sees that your praises and prayers are defeating him every time, he'll think twice before tempting you with negative thinking--and you can be thankful for that!