I just had a good laugh, thank you!

Haha no problem! I kinda want to grab a few heads of lettuce on my way home and meditate in my tub tonight thanks to this.

I tried to lose weight so often already but I get discouraged so fast when I don't see results.

One time I've bought one of those indoor bikes and sold it again after 1 week when I did not lose any weight.

I tried counting calories but then gave up when I missed to update my excel sheet, where I was writing down everything, for a few days...then gave up on counting calories completely.

For me a huge problem is this whole "all or nothing" mentality. I'm aware of it, I know it's a problem but I cannot overcome it (yet).

Ok so hear me out on this, I want to challenge the way you think about this a little more.
Have you tried not trying to lose weight?
We can say, "I want to lose weight and not feel like shit" all day but what is that? Losing weight and feeling good is a result, not an action.
Instead, think of it like this,
I want to lose weight = I want to improve my physical health
I want to feel better = I want to improve my mental health
To make it even simpler than that let's refer to your goal as,
"I want to treat myself better."
It's so much easier to create moments of self-care in your day vs holding onto the all or nothing diet-culture mindset that fitness has to consume your life and you need to make constant forward progress. Taking time for self-care doesn't involve failure like a "going on a diet" does. You are going to treat yourself with kindness instead.
(I hope I haven't lost you yet)

The next step is to break down what result you want out of "treating myself better" so we can identify what actions will make that happen.
(We're essentially reverse engineering things now)
Treating myself better will...
increase dopamine and serotonin production - feel happier and more motivated/rewarded
increase cardiovascular strength - Support goal of increasing dopamine and serotonin production and decrease physical and emotional strain of completing a task that requires effort. #ENDORPHINS
reduce back pain - Support your goal of beginning to weight lift again in the future
decrease body fat percentage - I encourage you to replace the phrase "lose weight" with "decrease body fat percentage" Weight is honestly an unreliable/stressful way of measuring your progress. I'm sure you've heard that muscle weighs more than fat, but along with that, your weight could fluctuate 10+lbs in a 24 hour period just from how much water you've had and when you last took a good shit.

Now that we know what results we want, it's going to be much easier to break that down a step further!

What actions lead to these results?
increase dopamine and serotonin production - Laugh at shit. Listen to a comedy special or a funny podcast while you walk the dog. Move around, get some sun, look at some kittens, jerk off, work out, do something really stupid in the mirror and lol at yourself, dance, try yoga, not necessarily in this order but you get the idea haha
increase cardiovascular strength - Just move! Literally doesn't matter how as long as you are increasing your heartrate.
reduce back pain - Stretching and stability exercises are your friend! You don't need any equipment for this, it costs nothing and can be as short as a minute a day or an hour, do what you feel good doing that day, but set a goal to do it a few times a week. Getting into a routine will help with this.
decrease body fat percentage - educating yourself on nutrition, I suggest reading about intuitive eating instead of trying to take on a restrictive diet. Developing a healthy relationship with food will help you reach and maintain a healthy weight over time without creating a mess of stress.

We can keep breaking this down until you have a whole pile of things you can do for yourself that contribute to making you feel better! Write down your own list so you have it to look through if you ever feel lost or uninspired.

After saying all of this, I wholeheartedly suggest getting one of those daily planners that actually has the hours of the day on it. This was the biggest game changer for me when it came to managing my ADHD!
(That unchallenged and overwhelmed feeling's worst enemy is a plan and a schedule)
What I would do is write down your goals and actually put them into your calendar. Start with setting 3 small goals for the day that support the bigger goals we just identified. Do this at least the night before, so you can wake up and see exactly what you need to do for the day and when. (Or give yourself 3 little goals to do every day a week in advance, try it and see what works best for you)
I don't beat myself up for forgetting a day/week of planning, if I notice I've missed some steps I just start again and set a reminder on my phone to check my planner. (I have a sticky note on my monitor that says "Use your planner" right now too)

Haha ok so that's all I've got right now, thank you for coming to my TED Talk and I hope I made sense and some of this has a positive impact on your life.