You have a number of ideas which prevent you from succeeding in the management of your weight problem. In order to succeed, you need to exchange those ideas for more constructive ones.
You write, e.g.: "I am probably not the kind of person who can succeed." By thinking like this, you hypnotize yourself into believing that you cannot possibly succeed. However, the fact is that we can change, develop and teach ourselves to function better. If you do it step by step, you can gradually bring about large changes.
Concerning your eating at parties: you can encourage your friends to offer better food by learning by how to make non-fat delicious food yourself and inviting them to eat it. But even if you eat fattening food at parties, it isn't so serious if you eat wholesome food other days.
You write that life is unfair. It is true but you get no benefit from thinking about this. After all, everybody has problems and it is pointless to blame life; concentrate on your own problems and try to solve them.
You don't want to appear impolite by leaving food, so in the first place you should restrict the amount you take. If it is still too much you can be polite by being friendly in other ways.
If fasting for one day a week doesn't work for you, then try some other method, e.g, having meals when you are most hungry, eating food which is satisfying and nutritious, and step by step leaving bad habits such as eating junk food.
You are afraid that you won't be able to manage your job if you slim. Are you sure? Isn't it so that bad eating habits make it more difficult? Naturally, you shouldn't slim excessively, but if you have good eating habits and don't eat too much you will probably find it easier to work.
You weigh so much that you think it will take a very long time to go down. There is an old Chinese proverb which says: a journey of thousand miles begins with a single step! If you get going with better food and find that you feel better and begin to lose weight, you will experience that as a success in itself. You don't need to wait until you are slim before enjoying the benefits of your new lifestyle.
You come with many predictions and if you are to be successful you need to test them and change the ideas you have. Try to consider what functions best in a practical way. Pay attention to your way of thinking, especially to negative ideas, and formulate a number of positive contradictions. Get used to doing this. For example: "I will never succeed and I can just as well buy a bar of chocolate on the way home", can be exchanged for: "of course I will succeed and I will buy an apple and sugarless chewing gum on the way home".