Hi Sassy,
I quit a 180k a year job two years ago, with six months of expenses in the bank, to start a consulting company. I did it in order to improve my quality of life (travel less, see my family more) and because I wanted to try to build a successful company again. I had tried this in my 20s and didn't really know enough to do it successfully.
Now I've doubled my income, with minimal travel and a 40 hour work week (down from 60-80 as a FTE). So, it was a great move for me. But the first year was really tough, and if I was awake, I was working.
You say you're a planner - that's great, that will help you out tremendously. Here's the steps I recommend:
1. Know your budget and exactly how much you need per month. Cut it back as much as possible.
2. Save at least six months, ideally 12 months, of expenses. Tune this to your industry, how long will it take you to be self sufficient as an independent? Some businesses need more ramp-up time, some are profitable out of the gate.
3. Have at least one paying customer before you quit.
4. Set a minimum stash that you will allow yourself to get to, before you pull the plug and go back to work for the man. Some clients pay late or never. Net 30 terms are standard for my business, you may be able to get away with better or worse terms. More stash means more insulation from late payments.
In my case, #4 really made the difference between success and failure. At one point in year one, I spent two months without a contract, and I sold my ass off. Due to delays in payment, we were within less than a month of our "go back to work for the man" amount. I really didn't want that to happen, so I worked extra hard to make sure I could keep going. Ultimately, I got a new contract, the old client paid (six months late) and it all worked out.
I also recommend writing a business plan - the score.org website has some great business plan templates that will help you think through your customers, growth plan, marketing, rate setting (1/1000th of annual salary per hour is a good starting point) etc. Make sure to account for health insurance, life insurance or other benefits you want to retain, paying estimated taxes (and the higher self employment tax you'll pay as an independent), liability insurance, and setting up a solo 401k for your new retirement planning.
The other piece of advice I have for you, is that the security of a full time job is an illusion. I've found that independent work is much more secure, because I control whether I have a job or not, instead of some faceless executive I don't know. There is no job security in the modern world except for the marketability of your skills. I appreciate being paid for every hour that I work, instead of endless unpaid weekend work to make up for someone else's mistakes. Once you understand this, it will be tough for you to continue working for the Man.
Good luck and let me know if I can answer any independent startup questions for you.