Well, I'm not at the '150 lbs or less' mark that you have set. Maybe I will one day... I've been fat all my life and have no idea what 150 lbs would look like on me! For my height (5"10) it would likely make me thin, but not too thin. I'd definitely enjoy seeing that number.
But regardless, I think I'm qualified to answer your question because I've kept off over 100 lbs for over a year now. I've lost about 130 lbs all together.
I can't really say if it's more difficult or not because I only have my experience. Just like people who are dieting with some obstacle (disabilities, complicated medical history, depression, diabetes etc) people who have been fat their whole lives face certain challenges. Namely, that we can find it difficult to imagine being thin because we've never been there, and maybe also because physiologically our bodies are comfortable being fat due to more fat cells or whatever the case may be.
But it is possible to lose weight and keep it off because I've done it and so have a lot of others on this board.