Tired of the roller coaster effect...

Try some yoga and/or meditation.

Choose foods that are calorie sparse rather than sense, so that you can eat more and feel full, and don't keep things that tempt you to overendulge in your home.

Ya, it takes willpower to resist some things, but you are a grownup. If you really want to lose weight, or even just maintain, then stick with that decision and find a system that works to keep yourself in check.

Try a different approach? If portion control and and eating sweets is a challenge, try the opposite approach?

For me, it was a matter of completely overhauling how I ate. "Will power" isn't really involved for me. I just eat a particular way. With the exception of a bit of dark chocolate a few times a week, and the very occasional no sugar added ice-cream or pudding (less than monthly), "sweets" just aren't part of how I eat anymore. And I don't miss them.
I used to have several Dr. Peppers, a donut, and a snickers each day, along with lots of fast food.
It started with me going cold turkey on all of that stuff, and then building a new diet from scratch. But it also started with me KNOWING that I was starting a new way of life, not just cutting stuff out to lose some weight.

Regardless, find what works for you and what doesn't require so much willpower. Best of luck.

We would be millionaires if we could solve the motivation struggle!! I lost 88 lbs about 5 years ago on Weight Watchers. I didn't quite make my goal, thought I was cured, and played the "you only live once" game. I have slowly gained about 65 lbs back.
I have tried to get the motivation back that I once had when I was dedicated to weight loss. Every week I battled with myself over and over again. Feeling guilty for eating this, or that. Feeling disgusted with how I felt in certain clothes, and wanting to bury my body in loose fitting sweatshirts, and sweatpants. Being mad that I can't eat, what I want, when I want!
I have recently, and when I say recently, I mean in the last 10 days, gotten my motivation back. I bought a Fitbit at Christmas time, and I'm walking daily at work, on breaks, and lunch. I have decided to make sure I walk, at least, 10,000 steps per day, Monday thru Friday. I am also tracking what I eat daily.
However, this does not stop that haunting question in the back of my head..."what if I lose my motivation?" The key is to not give up! Even if it takes some time to gain the motivation back. Celebrate the little steps you make, those non-scale victories that WW teaches. When the slice of cake you take is smaller than your normal slice. When you say no to that candy bar staring at you in the check out isle. When you opt for a salad instead of a burger. When you make a healthy dinner, and feel satisfied after you've eaten. When you have meal planned for the day, and made your lunch for the next day.
Stop beating yourself up & focus more on the good than the bad. Treat yourself like you would a friend. Read stories on these pages, and allow yourself to be inspired.
I wish you the best of luck & both of us long term motivation! We are SO worth it!!

Everyone would love a quick fix, alas, there isn't one. Frustrating, but unavoidable.

If you're having issues staying with a plan, try starting slower. Maybe make one adjustment, give yourself a week or two to get used to it, and make another adjustment. It is slower, but I know for me it helped that I didn't just change EVERYTHING at once and try to maintain it! Doing it a little at a time kept me from feeling deprived of things I like and helped me build a foundation for where I'm at now. I still have a horrible diet, truth be told, but I did make adjustments to get the nutrition I needed, can stay full for the most part, and still enjoy what I eat.

Also, motivation? Best gained from yourself, not others. External support and motivation are nice, but sporadic. Take a good, honest look at why YOU want to lose weight and make that your motivation. It doesn't have to be 'to get healthy' either. That wasn't mine. Once you find that, keeping that reason in mind will be your best motivation.

Also also, you will make mistakes. Learn to forgive yourself for them. When you make a mistake, log it, learn from it, and start fresh next meal/day. Occasional mistakes aren't going to sabotage you or make your a failure. Just makes you human.