This is the ultimate vicious circle of relationships: the more insecure and clingy you get, the more they'll pull away, and the more you'll try to hold on. It's the delicate balance we all face, especially if we're confronted with the thought that they might be cheating. If you're insecure, they might back away and start actually cheating. If you're not, they might think they can walk all over you.
Sounds confusing? You're not wrong. You could be the manliest man and the most independent punk in the world, but you're still vulnerable to desperately clinging on to love. Basically, you've got to watch your own actions this time, and listen to your brain. It's the one making you clingy in the first place, because it knows, deep down, that there's something wrong in the relationship.