No, unfortunately you don't have to win only once. For most people who struggle with obesity, it's a lifelong struggle. Jojo effect, mental state, hormone changes, other illnesses, etc. I was on a good way but gained 20 kg when I stopped smoking. Stuff happens, systems fail and stop working, even after they worked for some time.
Shit sucks. Which is why even temporary successes are great and to be celebrated.