Yes, I can relate.
If I change directions, it is like the first course of action no longer even exists. It's history - time to move on. I think part of it is just a logical response to the decision. If I have decided to go in a new direction, I can't spend time thinking about what I used to be doing. That's why it can seem like going "cold" on the old thing very quickly.
Really, I think it is a form of self-preservation. If I can't go full-swing into the new direction, I am going to be second-guessing my decisions at every turn. The going "cold" on the old thing reduces anxiety about pursuing the new thing and also conserves emotional resources.
This happens with career/interest changes, as well as failed relationships and painful experiences. I have a saying: "Put it out of your mind and never think about it again." It's like these things get encapsulated somewhere in my mind like a pearl in an oyster. Sometimes, they get aggravated and come to the forefront of my mind, but mostly they just stay hidden and ignored, because if I think about them I will get very upset.