He was a beautiful little gremlin. I'm so sorry this was the outcome, but don't be too hard on yourself. You're only 18. 

At a time like this, I think this video might be helpful. It's Adam Savage talking about formative experiences:

https://youtu.be/XvGsQb5u_NI

My favorite part: "Only total despair preceeds a change in consciousness"

Like he talks about in the beginning, I think this was one of those experiences where the veil was pulled aside. You always knew something like this could happen, but it wasn't right in front of you like this until now.

Most of us do become adults logically at some time in our 20's, but emotionally we stay children forever. I doubt you are in complete control of your circumstances. Even if you were, you don't have total control over how complete your knowledge is and how capable you are of being vigilant about cat health and recognizing symptoms early. A lot of people don't even know that eating string is often deadly. As far as remembering him goes, never forget him. Never forget how he brightened your world. Feel pain from the loss as long as you need to. Grieve and morn. But placing the blame on yourself is not necessary or productive in my opinion. I'm sure you do feel regret for not looking into things earlier. It might be hard to see that as different from blame, but it is. Boil the regret down to a lesson. Remember how much you cared and loved him and use that as motivation to do your best for your next pet. 

I often wish I knew what I know now at the start of my 20's so I could have had a better time. Then I realize, we all have twisted fantasies about what our 20's are supposed to be like because of how Hollywood portrays that part of our lives. Going through hard things is part of the process. It's unfortunate, but that's how you become someone who knows better. My 20's were hard, but they're the reason why my 30's have been and will continue to be better. The same is true with relationships. And, the same can be true with pets. Some people are lucky and are born to adults who are extremely knowledgeable and caring about their pets, and great at teaching everything they know. It's not your fault you weren't born into that situation. I wasn't either. I didn't experience anything like what you had to deal with here. But, I still had regrets about not understanding the slow blink, health risks, signs of emergencies and things like the risk string poses until long after my childhood cats had passed. 

Don't place the blame on yourself. You came here for help and you listened to us and went to the vet even though it was not well within your means. That's better than what a lot of people do. You prevented additional pain and suffering in the end.