In Act I scene 5 of Macbeth, Lady Macbeth gives herself a pep talk where she tells herself that she needs to shed all qualities of a woman she may possess. She needs to do this in order to help her husband become the man he needs to be, which is King of Scotland. When the murder is committed she is strong and actually assists in the finishing touches. However, guilt sets in and it ends in her demise. It is not that she cannot handle the pressure of being queen it is the secret that she keeps as to how she became queen that becomes the burden. In the end, Lady Macbeth does not enjoy being queen as much as she thought she would. So much time was spent keeping the secret that there was not time to actually enjoy the fruits of her labor.
The significance of Fleance's escape it that the witches prophecy for Banquo (that his sons will become kings) could still happen. This foreshadows that something will eventually happen to Macbeths reign over Scotland.