I haven't killed myself because I still have things I want to do. When I've completed my to do list, I have an end of life plan.

Because life can be a lot of fun.

Why would not believing in a god make life meaningless? Better question, how does belief in a god give you meaning?

The simple truth is that people find meaning in a wide variety of things. For some it’s the things they accomplish, for others it’s the relationships they build.

I cannot speak for all atheists as there are no common beliefs shared among us. Personally I do not fear death, as I do not believe there is an “other side.” I choose to live because I like life, and want to improve the world for others so more people can enjoy their lives.

No. You are engaging in the false dichotomy that the only alternative to externally bestowed meaning is no meaning. I lead a very meaningful life because I know how to find intrinsic meaning for myself.

If there is a God, and hence a heaven, is the reason theists don't kill themselves that they don't really believe in either one?

Here's a little historical note. Without the bolted-on suicide taboo, Christianity would have become a suicide cult. Think about it: life is meaningless. But god comes along to provide meaning. But we don't understand the random sorrow and pain that comes our way despite our belief in god, and won't until we're in heaven where everything will be perfect (and perfectly explained). Given that, and assuming you truly believe it, the logical course of action is to leave this veil of tears immediately and go to the bosom of your lord and savior.

The Donatist sect in early Christianity believed that they could attain martyrdom and go directly to heaven by killing themselves. The suicide taboo was largely concocted by Augustine's 5th century City of God to correct for nuisances of that kind.

Ffffft. Why would I want to kill myself? I happen to enjoy life! Has nothing to do with fear. Aside from that
my life has plenty of meaning without some wacky religion dictating what that meaning should be.