It can be tricky, can't it? I tend to agree with most everything you've said about religion - mostly a way to make the weak feel somehow empowered, those without control to feel things are being controlled. It has been accurately described as the opium of the people. So what is one to do when one kicks the opium habit?
I was lucky, my mind has the tendency to wander and I always spent a lot of time (even while in the cult, and I suspect this was a big part of my eventual departure from it) contemplating hypotheticals - including things like "if god doesn't exist is there still any value to morality" or "if there's no afterlife, is life worth living." Things like that. So by the time that I actually realized I was an atheist, I'd already pretty much come to terms with all that entails.
One thing in your post stood out - the characterization of life as pain punctuated by happiness. I think a lot of life is about your outlook. If you really think your life is mostly pain, then you need to start looking at the reasons why and addressing them as best you can. Maybe your life does suck, but maybe it's just that you're focusing on the wrong things. If, as you pointed out, all there really is to this life is trying to enjoy it and not be too much of a dick to everyone else, then there's no time like the present to start trying to make your life look like you want it to. Also, while you're taking action to that end, you'll have less time to ponder the meaning of it all which would probably be beneficial for you at the moment.
I also would like to state that I wholly reject the notion that the brevity of life makes it somehow not worthwhile at all. When I buy a brand new car that I optioned just the way I wanted it and I'm inhaling that new car smell, I know somewhere in my mind that this car will one day be in a junkyard crushed and shredded for recycling. Should I not have bought the car? Is the car completely valueless because of that knowledge? Chicks love it when you give them flowers, even though they will be in the trash in a few days. I can enjoy a meal even though I won't be eating forever. Brevity may limit the value that something has (i.e. a car that will die tomorrow is worth less than one that will last 10 years) but it doesn't make things completely valueless.
Regarding the notion of "nothing you do matters, so what's the point?" - this is largely up to you. IMO we all have the freedom to define the meaning of our lives. You don't have to pick one thing. You can devote your life to a family, to having new experiences, to developing some skill or talent, to learning new things, or any combination of any number of things. You don't have to look to anyone else to tell you the meaning of your life. If the purpose you chose isn't doing the trick, pick something else. Just keep going as long as it's fun, then when it stops being fun figure out why and find a way to make life fun again.
Also, I would like to point out that a little bit of nihilism can actually make life a lot easier. When you screw something up, just remember that nothing really matters anyway. When you get something right, maybe it doesn't matter but it still makes you feel good, so who cares if it doesn't matter?
I would definitely encourage you to try to ease off the drinking and find some hobbies or something that keep you interested. Remember that there's no rush, and you don't have to figure out this existential stuff right now, and it's probably not super helpful if that's what you devote all your time to. Find some things to put some energy towards and you may find that the existential crisis resolves itself as you realize that every day is worth living on its own merits.
And for some levity:
