Most people would get pretty overwhelmed by saying they'd like to lose 100 pounds, or even 50. It's not easy to see how you can ever reach that point when you're at the beginning.
Something that might work better for you is to set a goal to accomplish something you aren't able to do now, but that you will be able to do once you're healthier. For instance, you might set a goal to fit into the jeans you wore 5 years ago. How about a goal to run 2 miles without stopping for a break?
I just read an article about a woman who set a goal of being able to wrap a bath towel around her body. After being able to do so, she found out she had lost 107 pounds! If setting a goal of a specific weight loss number is not working for you, try another tactic along these lines.