I think the reason you drop your motivation is learned helplessness. In an experiment a dog that’s been given electrical shock had no where to escape. So he learned that whether he tries or not he’s gonna be exposed to the shock. Later when he’s given the opportunity to escape from the situation he doesn’t move a muscle because in his inner mind he thinks there’s no way out. So it’s just your illogical subconsious mind that causes you to give up even when you’re capable of many things than you think right now.