I need a kick in the butt

I don't think there's a solution, other than just committing to getting back on track.

It's kind of common to experience going off plan then struggling to resume. I'd encourage you to think about whether what you'd been doing when good was maybe more aggressive or extreme that it absolutely needed to be in order to gradually lead you to your goals. Having a harder plan can make it harder to get back on track.

Also I think it's normal to feel like we've ruined all our progress with one rare day with an indulgence, and kind of mentally give up. While the scale may go up the next day or few, that's mostly from water weight in most cases, because of the change in types of food: It's not fat regain.

Somehow, if we're going to not just reach goal weight, but stay there long term, we need to find tactics that allow us to have a normal, happy balanced life. IMO, such a life includes things like having some cake on our partner's birthday.

For myself, when I was losing weight, I tried to plan ahead for predictable, scheduled things, and eat a small number of calories under goal for a few days beforehand - like maybe a hundred calories or so - and bank those for the special day. I'd plan to eat lighter than usual at other meals that day, maybe fit in an extra walk or something if feasible. YMMV, but to me, that "plan ahead" works better than "I will refuse birthday cake", because cake is tasty, and part of the social experience. The long-term impact usually isn't huge from unusual special occasions like that, when looking at the calorie arithmetic, even if it's unplanned.

IMO, part of a good plan is recognizing that even the best plan will break down occasionally, because life is complicated, and we're human. Committing at the start to get back on track after off moments - maybe with a plan revised to better handle similar circumstances in future - is also part of a good plan. The sooner that "back on track" can happen, the better.

I'd emphasize that it's our routine habits - things we do day-in, day-out on repeat - that are the power tools for weight loss. A rare day with too much cake, or a rare day with a 5-hour workout, is a drop in the ocean. Our routine daily habits are the ocean, when it comes to weight management and nutrition.

You can do this. Adjust your plan - make it easier, more realistic, if you can - and resume. Persistence and perseverance are important. Suffering is optional. As long as we keep going, keep making our plans more doable and realistic but still positive . . . success is out there. It's giving up the effort entirely that doesn't work out well.

Wishing you success!

If you have an addiction to sugar/carbs, then a taste of them can be like cocaine to an addict. It can get you hooked again. You will have to wean yourself off sugar by going strictly low carb for several days until the cravings disappear. Been there, done that.