Motivation to diet and lose weight can be affected by many things. sometimes hopelessness plays a part, sometimes disappointment, and sometimes even just being tired can get in the way.
We tend to think of motivation as a particularly magical mindset that some people have and some people don't. It really doesn't work that way. Motivation can come and go for all of us - it can be high at times, it can be low at times.
Really sticking with something may take just a little extra thought and consideration, digging just a bit deeper, reaching just one more level of understanding beyond where you are now.