It's totally normal to feel sad when a big project like this is nearing completion. Although it's hard to beat the excitement of seeing the scale numbers going down, it sounds like it's time to get creative and come up with a new project that you can be just as proud of. While weight maintenance is a formidable challenge in itself, it does not make for a very exciting goal.
I like to introduce specific goals like personal record time/achievement at an athletic event. Or, eating a set number of veggies/fruits every day (which you probably already do).
Or, go with something totally unrelated to fitness but still very measurable, like saving money and growing your retirement accounts. It's pretty darn exciting to see money grow :)
I hope you find something exciting to keep looking forward to and always stay positive.