Hey
@Rabbitthekitten! Greetings from another chunky martial artist. I started training roughly 6 months ago after 16ish years away from karate, and I weighed far more than you (and still do). I walked into the dojo at 340 pounds and I am presently 300. I still have a long way to go, but I've dropped over 10% of my body weight, which is pretty cool.
MA alone certainly hasn't gotten me this far. I do some walking and hiking too, but the most important thing is diet. As several people have said, pretty much any diet will work in the short term. The problem is sticking with it for the long term. I like my diet. It doesn't really cut out anything. I eat french fries and pizza when I want to. I also eat a lot of chicken breast and veggies.
Motivation is always a tricky thing. I'm pretty motivated to go to my karate classes even if I'm tired or feeling lousy (And honestly, that's a lot of the time). We're pretty much a karate family, and my absence would be noticed. Plus everybody is rooting for me on my weight loss journey. I also have an invitation to go for a plane ride in my instructor's two-person plane when I get down to a weight that's safe for the plane.
I also know that if I start slacking off, I'm going to really suffer in the classes I do go to. That's pretty motivating, because the first month or so going from couch potato to 3+ days of karate was rough. I don't want to regress! I'm not sure I have specific advice for motivation, but I will say that the longer you stick with your training routine, the easier it is to stay motivated.