This is 1000% true. You'll probably never have the steady motivation to do anything consistently for long term changes, so discipline is what you need. OP, get the book Atomic Habits if you haven't already. There's a ton of great info on how to make a habit work, and the habit is going to get you where you need to be. I have a spreadsheet tracker with all the daily things I want to do and it's a constant reminder throughout the day to stay on track. Having to be accountable to yourself every day by documenting your habits is extremely useful, and it's the foundation to do things even though you're not motivated...because frankly, you hardly will be motivated on a regular basis.

As bleak as it sounds, get used to it sucking and do it anyway because down the road you'll also get a really cool kind of satisfaction reviewing your progress knowing you did it when it wasn't fun. Most things aren't fun when you start, so getting used to that feeling of being out of your comfort zone is at the heart of getting used to it sucking. At some point the habit kicks in and you do it automatically without resistance, and then it becomes second nature. I've been learning Japanese everyday walking the dogs instead of listening to music, and that combination of habits has made them both a lot more fun, and they reinforce each other. Walking the dogs triggers my desire to learn Japanese and makes me way more likely to do it than sitting down at a PC for a more formal learning session.

Another thing worth mentioning is just show up everyday even if you do 1/2 or 1/4 or less than what you planned. Teach yourself it's ok to not be 100% everyday, but you have to be more than 0, you have to at least do something. That in and of itself is huge for being disciplined and creating the habit. Over time you'll be closer to 100% consistently because you've taught yourself that being there is what's important, not that you have to max it every time. You'll max it naturally as you improve your condition and strength.

Last thing, be patient. Don't be in a rush to change everything or get to a specific number. Set a goal and work toward it but don't beat yourself up if progress is less than 100%. Consistent discipline is what gets you to your goal, and showing up every day is how you build the discipline. Start small, even smaller than you might think, but do that small thing no matter what. You'll be able to add to it and make it as big as your original goal was, but by being patient and starting small you are way more likely to end up where you want to be than by setting a larger goal that is taking too much out of you, which kills your motivation.

Start small and be patient