Your primary goal should be your health. Having large amount of excess fat proved to be hazarderss to your health in many ways externally (social, appearance, etc) and internally (heart, etc).
What you should do is have small goals, instead of big ones. Like going from 300 to 185 is a big goal, but going from 300 to 285, is a small goal. Once you reach that smaller goal, you can set another small goal from something like 285 to 265 and so on. Smaller goals will help you get bigger goals.
I guess you should know that just exercising will not allow you to reach your goal, no matter what. Because you are burning excess energy, you might start losing weight. However, as soon as you stop, you gain the weight back. You want to know why? It's your diet. So your goal should include diet and exercise, instead of one or the other.
Personally I hate the word "diet" because society has made that term into "starving yourself" or "eating less." You should eat right, not over or under. So think of "diet" as "lifestyle change" rather than "starving yourself by eating things you cannot possibly eat 6 months from now."
As to answer your question on how to keep yourself going, that's hard. Most people quit after a week or two of exercising. I have seen several people come through the gym thinking they're going to lose weight by running really fast for 2 minutes. They're all gone because they probably realize just how hard it is to lose just 1 pound by doing pretty much nothing but burning yourself out. The best thing for you to do is get yourself in a schedule of some kind. Perhaps keep a journal of your diet and exercise so that you can keep track of what you're doing. Try setting up days where you spend time doing one exercise whereas the other day you do something out. Just doing the same thing over and over with little or no result will make you upset and quit. Come back to the forum and show us your results. This will help you and keep you motivated. Talk to your friends and family and tell them how much you lost each week. You should measure your results by months, not days or weeks.
Also, you should note that everyone is different in how they lose or gain weight. Your genes will tell how quickly you can burn excess fat and gain muscle. You can reprogram your genes to change how it burns fat, stores muscle, and use energy by dieting and exercising the proper way. Of course, don't blame your genes if you can't lose any weight at all. Your body is completely under your control and what you put in it, will tell what will come out of it.
I personally started my quest two-three months back. I lost weight, but only time will tell if I'm successful with what I'm doing.
Good luck.
