You may be overcooking them. Make them smaller and cook for less time. Another possibility is that you're squeezing the bejaysus out of them when you make them, for fear of their falling to bits. Roll them in your hands without pressure and employ an ever-so-gentle touch. And blimey, all those herbs and spices. Don't you trust your meat? My way is to make them small, fry them in oil just to brown them, maybe for five or six minutes, not cook them through, then let your chosen sauce finish the cooking, maybe for ten or fifteen minutes. For me, it would be a spicy tomato sauce made with extra virgin olive oil. By spicy, I mean with chopped garlic, no onion and some chilli flakes to taste. I always tear in some fresh basil leaves at the very end. That goes really well with crusty bread, or some home-made oven chips. I'm not a fan of spaghetti meatballs...

And leave the breadcrumbs in the store cupboard. I've tried meatballs made with a mix of minced steak and crumbled black pudding, or a mixture of minced steak and minced free-range pork. A touch of chilli always goes well. One I got from Gino d'Acampo is beef mince with a bit of caramelised onion chutney mixed in. It sounds like it shouldn't work, but they are delicious. I usually find that you need a tad more salt in the seasoning than you might expect. Small is beautiful.