Recent research shows your mental outlook on weight loss or your perception of weight loss is just as important as your eating habits and your exercise prescription. It's not "wishful" thinking. Whatever is going on in your mind is what you are going to get.
People think about what they do not want and get more of the same. If you start your new fitness plan by saying "I want to lose 10lbs but if I only lose 5lbs it will be ok" the you are already setting yourself up to fail.
To get started, think about how you can identify specific weight loss and exercise goals and plan to avoid some of the most common mind-set pitfalls. What that means is, if your goal is to weigh 150lbs, why is it 150lbs. What have you tide to this number. Do you really want to weigh 150lbs or do you want to look a specific way, a certain size, muscle tone and so on. You need to have a clear, vivid and specific picture of who you want to be and what you want to look like.
It's not that some fitness goals are unachievable or unrealistic. It's more the fact that they are undefined. They are not clear so you never meet them. Then not only will you suffer trying to meet them, the backlash you'll feel when you do not meet them leave you down mentally, and sometimes so unmotivated that you may not even want to try anymore. The weight did not appear like magic, so do not expect it to go away overnight just because you want it to, focus on what you want, develop a plan to help you achieve your goal and take action.