Pushmeinthepool
You have every reason to feel resentful of your parents, but if you are going to break the pattern you describe, you are going to have to recognise that it was not just the things that were done to you by employers, parents, exH, but also the way you handled them that got you here (albeit your lack of confidence is completely understandable given you controlling parents etc...)
You say you were never 'given any chances to have a career' but I think you may have overlooked a lot of chances along the way because of your lack of confidence and thinking that the only way into a career is with qualifications. Of course there are vocational jobs with defined career paths, Doctor, nurse, lawyer, teacher etc... but many people nowadays make their own career by being pushy, spotting opportunities and jobs that needed doing, putting themselves forward, act resourcefully etc.. and lots of them start off in admin positions. Those who are successful don't wait to be 'given chances'
Most jobs have a fair degree of administration in them, and in my experience administration can lead to all kinds of avenues and management skills - managing projects, people and budgets, managing events, dealing with legal, communications and PR, print buying, web design, office management, HR, IT management, facilities management, basic research, running meetings, purchasing. Did you do these things, and you are selling your self short, or did you not see that there were opportunities to do these things if you pushed yourself forward?
When you started the negotiators job, I suspect your boss saw your lack of confidence and had a gut feeling you didn't have the spark of a negotiator. The fact that your response was to quit rather than to negotiate your position probably confirmed that to him. When you had the day in the office typing up property details it sounds like you saw it as a demotion, and went into resentful mode rather than trying to work out if you could use it as an opportunity to use your skills to help the company to be more successful. What if at the end of the day the boss had said to you we would like you to do the admin role and you had said 'OK, but since you are now hiring me as an experienced administrator rather than a trainee negotiator on commission, I will need a higher basic rate of pay in the range of x. And by the way in my day in the office I noticed these things with the IT system, work process management, office systems etc.. that could be improved and could save you something in the region of x, or bring in new business etc..'. Things might have turned out differently (admittedly they might not have but it is always worth giving it a try 
).
At 34 you are not too old to kickstart your career, but you are not getting any younger either, and you have the disadvantage compared to 21 year old career starters that you are not able to drop everything, move to a different part of the country, work late and flexibly etc...on the other hand you do have advantages of a mature attitude and reliability, administration skills and experience and the financial security to be able to volunteer, do something low-paid/take a risk if you see an opportunity to progress.
My advice would be not to write off your experience in administrative roles, but build on it. Sit down and think of all the stuff you have done- delivered time sensitive projects, managed people or money etc.., talk it through with people and on Mumsnet then do a really targeted job search, what you want to do is find a local company or organisation that needs your skills and that would offer you the potential to progress. I would generally avoid big companies and local Council that tend to be less flexible. Anything advertised in the job centre will tend to be low risk/low return in career terms. What you should look for is something where you use your admin skills but gets you into doing some chunky project work in a sector you are interested in, not just filing and typing. It might be a start-up company, it might be a charity, it might be a school or in the health sector. Talk to everyone you know about their jobs and friends of friends. Don't look for a job with 'status' or high pay in the first instance but be wary of anything that looks like a dead-end job (I would be wouldn't go for saturday retail work for a chain, as it is rare to progress into management without being moved around branches, and if you don't want to do that then this isn't the way in for you).
Unless you are really set on a vocation I wouldn't go to Uni, it will take years and at the end of it you will be in the same position of competing against whippersnappers.
You will have to be pushy and resourceful to find the right job, and pushy and resourceful to make it work for you. When you start don't just try to keep your head down and do what's on your job description/in your comfort zone, look for opportunities to learn and to demonstrate what you can do at every turn.
Don't be too hard on yourself, but don't wait for others to give you opportunity.
Best of luck!