Deceit and defeat

I'm actually quite a happy person. I got a wonderful family, a smart brother and great friends and especially a magnificent God.
But I don't get why my mind is always filled with negative things. Most of the time, the negative things do not warrant me to feel bad at all. Yet, it oftens become amplified in my mind. For example, if I were to mistakenly teach others the wrong things, I'll be saying "You are useless. Don't try helping, you'll make things worst" and this kind of thoughts. Often I know these are from the devil and he is trying to defeat me but everytime I hear these thoughts, I'll become very depressed for quite a while. I try feeling my mind with words of truth but either because i choose to or for some reason, it'll not go away. Only after a long time or when I sleep then it disappears.
However, by then, usually the damage is done. During my time of depression, I often close up myself and not talk to anyone. Even my friends were to talk to me, I'll only give one word answers. Sometimes, I do tell them I'm depress but I rarely know how to explain myself plus i'm afraid that they do not understand so usually I keep it all alone, relying on only myself and hopefully God to do the healing.
I hate this. It's leaving me rather defeated and deceived. God, please teach me what to do.