hermione, do not lose faith. I know how people can be time crunched and I was in your situation last year when I had to crunch the GMAT with my CFA exam which made it that I did not even reach my target score after my second attempt. You will have to prioritize and as for this year's vacation, you know what to do with them

Yes, it will be part of the sacrifices you will have to do to reach your goal: no pain no gain. We all know that.
However, the good thing is that it is CLEARLY possible to increase your score from 540 to 700+. Some people have done it and you just have to go through the stickies in the "share your experience" forum for some such inspiration stories. Often times, inspiration is what will you that little needed boost to fulfill your dream. Of course, hardwork is also quintessential. If you have 3 months ahead, I would suggest you to dedicate about 1-2 hours a day during weekdays and about 6-8 hours on weekdends. You should do as many problems as possible, both from CATs and from gmatclub. Try to participate more. I once thought about joining a class with PR or Kaplan but in hindsight, that would have been a waste of money for me. Honestly, I learned more here than anywhere else. I joined every possible discussions and solved many problems, as you can see by my number of posts, and it really did pay off. If it can encourage you, the first time I did my GMAT, I did not even know that you had no right to a calculator until the day prior to the exam. Needless to say that I did not score good at all. However, I did achieve the 700+ in the end
Use your commute time to read a bit on the GMAT. Use your free time at work to answer a few problems from gmatclub. I was also studying for my gmat during lunch time

Yes, you will be living, breathing, eating and thinking GMAT during the next 3 months but the good news is how better you will feel afterwards when you will reach the mountain top. You do not need to be superman to do it, you just need to have the determination.
I expect to see more of you in the coming months and do
actively participate. Do not hesitate to ask any question or contact me and come back to share your story thereafter.