How do I stay motivated after gaining weight?

Track measurements, not weight.

My goals are behavior change, like "stick to 100-150 snacks this week", or "add a new activity midweek". That is totally under my control. The scale, a little less reliable from one measurement to the next, even though the overall trend is downwards.

If I get an unexpected gain, I focus on analysing what happened that week and brainstorming a solution. I work really, really hard to keep the emotion (feelings of failure) out of it.

It is what it is. The weight is going to fluctuate and will do it for a variety of reasons: not enough water, too much water, too much salt, no salt, canola oil vs olive oil, did you have peanuts or almonds, hey it's sunny outside!!

It's a process. If you're logging consistently, and doing things consistently in the right direction, have faith and confidence in what you're doing. I've had weeks where I gained 3 pounds, only to come back the next week and lose 4.5 pounds. No rhyme or reason to it....just happened that way. Do things in such a way that if you do gain a half pound that you can review your week and see if there was anything you could have done differently: are you sure you logged everything right? Did you get in enough water? What about fiber? Did you forget to track any condiments or sauces or dips that would have had an effect (mayo, tartar sauce, queso...)?

Use it as time to reflect on things that you can try next or do differently somehow.

I just try harder that day to stay on track. it's not easy but it's doable.

A number on the scale is simply a measurement of progress, the same as body fat %, the measurement of body parts in inches, the number of pounds one can bench press or squat, how far or fast one can run, etc. I believe that any one of these measurements should not be the end-all-and-be-all of a person's efforts to become more healthy and fit but neither should any of them be discounted. We cannot out-exercise a bad diet. Running and weight lifting doesn't make one thin but neither does getting our body fat % below a certain point make us healthy and fit.

The simple fact is, our society places a lot of emphasis on that scale number and it's one of the easiest and most obvious ways for a person to measure "progress". The number gets smaller, progress is being made, QED. There are TV shows built around people trying to make that number go as low as possible in the fastest amount of time. I abhor those shows. I think they've done at least as much to distort people's perceptions of what weight loss should be about as the diet industry has. I believe that another measure of progress is when one realizes that the scale number shouldn't be the only goal or even the most important one.

Weight loss is not linear. You need to wrap your head around that concept. Some days it's going to go up, especially as you get closer to your goal. If that number is such a big deal to you, find another measurement without as much variability and volatility and use that instead.