Hi Andrew,
I'm not entirely sure what you are saying. Is it that you are a theist who believes in heaven and so life does not matter? Or an atheist having trouble reconciling the finality of death? I am only confused because you say you have turned away from theism, but also mention that you don't fear hell. I find this somewhat motioning to a belief in hell that you just aren't afraid of.
If it is that you are an atheist struggling with "breaking up" with the idea of an afterlife, I would like to share my views. In my mind, our lives matter in that they end. The caliber of your character can only be demonstrated in the limited time you are here. The more you love, give, laugh, learn, and teach, the larger impact on our species you will have, and at the end of your life, the more you have bettered humankind. Each species strives to improve themselves, and mankind is no different. The meaning of life is to try your best to improve our species, however you can. Well, that is its meaning to me.
Were you to be immortal, through an infinite tenor in heaven, what value would you have? Everything you do, you could do infinitely over and over again. There would be no special moments, no once in a life time experiences. Because your lifetime would never end. You would not really even have a lifetime, you would just have time. You would have forever to accomplish everything. Furthermore, what would motivate you? What would be the point of doing anything if you can always just do it in another thousand years? Instead of living a short life to its fullest, you would live an infinite life stagnantly. There would be no meaning to your existence because there would be no end to it. Sure, one can involve the presence of God in their afterlife, but, one must wonder if his greatness fades once you are, for all intensive purposes, just as great and infinite as him (for i believe much of his greatness is derived from his infinity). You would both be everlasting, and I'm sure after a few million years his star status nulls a bit in your eyes. Were heaven to be true, you would live your entire life building up to meeting this one great power, die, reach heaven, gain the immortality you so admired about him, and then spend the rest of eternity with an old bearded man/eight armed elephant/spaghetti monster, that isn't that much better than you after all. If you spend your entire worldly life anticipating meeting god, what do you have once you meet him? A very long, very uninspired, very meaningless stay up in the clouds.
Anyways, that is how I place meaning to my life. It may not be the right explanation for you, and I wish you luck in your search.
Cheers,
Liv