Erasing selective portions of memory from human brain

Not all memories are pleasant. Some pierce your soul like thousand knifes. They say necessity is a mother of all inventions and I cant fathom why scientists have not figured out a way to delete unwanted memories. So I began reading up how human brain works. A memory is a combination of the object's name, function, shape, color, sensory perceptions of various sensory organs and lots of other characteristics. Each of these are neurons present in different parts of the brains, so takes unimaginable amount of processing to retrieve and combine them in a pattern that produces the same effect it did when it first happened. There are innumerable associations between these cells which defines how we react to various situations, or relates the past to present and sometimes defines the future. When one of zillion cell retrievals sometime fails, we forget some things or memories don't result in same kind of experience. Over time it fades. It appears that deleting a memory would involve deleting zillion cells and an equal or more number of associations. And it they define how we react, we could have serious side effects. So, in that sense all memories are good, while some make you happy, others train you about what not to do or how not to react so that you can be happy.