How do I know that it's my fault?

I often blame myself that it's my fault when things are not working well. It is easy for me to say I should have done this and that for almost every moment. 

My best friend in office asked me, how do I know? How do I jump into that conclusion that I am at fault? Yes, it may be my imperfection. But does imperfect mean faulty? Also, it may be due to various contributions from others. Or it may be just it takes time to do it right. There is no shortcut for it which means if I cannot do it, then I am a problem. 

Similarly, even if I lose job, does it mean that I am a failure? How do I know that? It may be just because of the economic situation. 

Another thing is how do I know that if I am not at the level or capability that I want, then it's my fault. This is another potential missing link. I often say that I am stressed because I cannot accept where I am now. But if that is the problem, what is the difficulty to change our mind and accept where I am? This again maybe because I jump into conclusion that being in my current state is my mistake. 

So be careful in assessing myself. Avoid jumping into conclusion. When I know there is something that can help, there is no need to judge that it is my fault of not using it earlier. We may simply forget and it is fine.