I am heartbroken, I feel like I ruined my cat....?

I'm so sad right now. I can't explain how horrible I feel. I'm sorry if I asked this question already, but I neutered my cat, and ever since he's been completely different. My cat used to purr and chirp whenever he would see me, and now he doesn't do any of that. And little things too, he never liked being outside in the dark, and now he roams during the night. He never liked going on the street, and now he does, especially at night. My cat loved ham before, now he doesn't want to come near it. Before he would get all excited when I pet him, and now he just sits still. He also doesn't respond to his name anymore, nor does he come to me when I hold my hands out, like he did before. I've been crying about it all day, I feel horrible. I feel like I ruined my cat, like this is a completely different cat, like his soul is different. And I wouldn't have done it, if he wasn't being attacked by the tomcats in the neighborhood. I never would have done it, if I knew he would completely change, and everybody told me he wouldn't. What do I do, I loved my cat so much, and I am completely heartbroken. It's like he feels nothing toward me, and before everybody would comment on how much my cat loved me.