First off, as you admit, it isn't that you don't have ambition necessarily, it's that you're not sure what you'd like to do, and do not want to do something that you don't "love." With that aside, you actually point out several issues you're struggling to deal with at the moment:
1) The conflict between you and your parents, in which you don't want to upset them by "doing nothing" but don't want to work at something just to have work;
2) A related conflict with your father, in which you don't want to "follow in his footsteps up the corporate ladder" while at the same time not wanting to disappoint him;
3) Your fear that you aren't "good enough" to do what you might want to do, something involving writing (though it isn't clear to me that you want to write - rather, you seem to be considering it because it might translate somewhat from your master's degree;
4) Your desire to work at a less "serious" job but your worry about not only disappointing your family, but also a potential love interest. These are the initial issues that got my attention, and believe it or not, they aren't that uncommon.
Another thought I had while reading your post is that if you will never know whether or not you'll love something unless you try it. This makes me wonder if you are also afraid to try and fail at something, whether it's a job or a relationship.
So with all that in mind, my advice is rather limited. First, you may want to ask yourself, what is the risk in disappointing your parents? That seems to be a big factor making you unsettled and discouraged. Second, you may want to go talk to a therapist about these issues more in depth. It can be overwhelming for someone to try to sort out all the things you're trying to sort out at the moment - relationships with parents, career path (particularly when you've decided not to pursue what you went to school for), romantic relationships, and most of all your own view of yourself. Psychotherapy can help you sort all of these things out and help you realize your potential, in whatever direction you decide to go.
Another opinion, hope these are all helping. Maybe you do have ambition, and you're current ambition is to not be like your father. It does make me want to question why when you have so many issues about how your needs differ from your family's expectations, you still live at home? Maybe a good move would to be to get a menial job, with enough income to allow you to move out and break the pattern of trying to please and also resenting your parents. Somethimes to move up you have to move down first. You are an adult now, and you are not answerable to your parents, other than in the love you have for them. If you want to work as for example a cleaner and use your free time to develop yourself or take evening classes in some interest, then do this. I just feel that while you are living at home you are still feeling like a teenager who doesn't want to be told how to live their life. You may never reach the aspirations of your parents, or you may reach higher, and you may find they love you regardless. But until you find your independance you will not be able to see these important people for who they are, with the same insecurities and failings as we all have, and maybe even as lost as yourself at your age. Yes of course seek therapy if it will help, but I somehow think your life may progress much easier if you live independantly. This world is a hard one, employers will only for a little while accept youth as an excuse for a poor c.v. You need to be back in the job market now, doing anything. As you get older the opportunities in work get fewer unless you have already proved yourself as employable. So take a menial job, maybe hate every minute of it, but stick with it to give you financial independance, self-esteem, and to open doors. If you really had no ambition, you would not have trained as you did, and you would not be experiencing the angst you now feel. As we get older so many of us learn that a lot of life is about doing things we don't like, boring things, with unpleasant people, but there comes a point where the payoff comes. You may have enough money to buy a dream property, you can impress the woman of your dreams, you can afford to take a year out and travel.... I am talking as someone who personally missed several opportunities in my early life, both in work and personal life, through being too choosy. Now I have become disabled and with serious health issues, which with my age makes it very hard to impress employers. If I had known I would become ill and disabled I would have done things very differently when I was young, taken jobs I refused, stayed with partners I turned away, many choices. Now my world is so limited. None of us knows what we have ahead, I never expected to become disabled. You need to grab your youth, health, and intelligence and take every opportunity that comes along, menial as it may be. Come the future, you will only regret the things you didn't do. The world is very much your oyster right now, and I think you can take that leap of faith and discover it.