<human>: Give me a shortened version of the text

I know many people would say, "just break up if you aren't happy." But I don't feel like it's that simple. 

We get along well. He's kind and devoted. What's more, we've weathered many hardships together as a team. 

The problem: 

I know passion in relationships tends to fade after a couple of years. But we've never had it in the first place. I don't think he's ever been passionate about anything. I used to be okay with this until relatively recently. 

I've felt a definite "spark" with a few other men in the past few months. I could never cheat on my fiancè, but I can't say I haven't been tempted. 

We can't have mutually engaging conversations, because our interests don't really overlap at all. His hobbies consist of playing games and watching other people play games on YouTube. I don't like playing games with him because I suck at it and would rather be doing something more "productive." He won't play music or make art with me because it makes him feel insecure. 

I've tried breaking up with him before, with the reason that I need more independence and want to know what it's like to be on my own. This was true, and still is. But I never stick to it, and let myself be talked into "working on the relationship."

Whenever I consider breaking up for good, I get seriously worried about him. He has no friends. He's a little emotionally unstable. I care about the guy, and I know that losing this relationship might upset him more than it would most people. 

Am I selfish for wanting to leave a partner that's perfectly good to me? How can I do this while causing the least amount of pain possible?
<bot>: My relationship with my fiancè has always been lukewarm, but friendly and companionable. I know I don't want to marry him. How do I break up in the kindest possible way?