How to deal with the concept of ageing and death?

"Live each day as if it was your last" A saying that has always stuck with me. While it's suppose to remind you that time is limited and you should aim to accomplish a lot each day, I fixate on the limited time aspect and can't seem to ever stop thinking about it.

It's true; we all meet our end. Doesn't bother me what comes next: either there's some spiritual plane where we all gather or there's nothing and if it's the latter, it's not like we're going to be aware of it right? Our sentience is gone at that point. One things for certain; we don't come back. so I think the best way to live life is to do everything you want to do while you're here.

What bothers me is WHEN it could happen. I try not to focus on the what ifs, but perfectly healthy people who: eat well, exercise and have no bad habits, can still drop dead from anything though granted not as likely as others. There's also all sorts of unnatural ways you can die and people with ill intent who just want to kill large numbers of people and you could be one of them. Even then, we all age a little everyday and will eventually become so old that we can't do anything; not even look after ourselves.

My biggest fear is my life being cut short; not being able to accomplish my "bucket list" because I kicked the bucket at age 30. Due to this constant fear, I've developed a negative outlook on life and can't enjoy it. I know this is a bad mentality, which is why I ask this question; I need help.