I know what you mean. This is my third try at a lifstyle change, and with about 10 months on track, its the most successful one.
My own personal experience is that I'll be great for a few months, and then for a month or two it will be more difficult. Motivation wavers.. as much as it sucks to say, that's really just what happens. You need to go past motivation, and tell youself "this is what I have to do." A trick I have when it comes to working out is that if I find myself attempting to talk myself out of a workout, I put down what Im doing and do it right that moment. (for me at least, trying to talk myself out comes as an internal debate when Im preparing to do it.)
You can do it!