I feel like I'm a bit like you. I got out of rehab and ended up not going to NA, I felt good and didn't feel like I related a lot to the general attitude there (and I still feel pretty good and have the same attitude to NA..) and the constant rehashing of the same old things. I hope no one who likes NA is offended because we all have different path to sobriety and I realize for many this is through NA/AA and that it's an outstanding program for them.
But I think I too got a bit low after awhile. Because there were things the newly-recovered me wasn't ready for the medium term-recovery period (I'm at 17 months off heroin now BTW). Such as;
1. I think the trajectory of recovery/life is VERY steep at first in sobriety, going upwards, but then it evens out more. You pour so much focus into getting sober, you have something to work at with concrete goals and you can physically and mentally FEEL yourself get sharper. You have something to celebrate almost every day. Then after awhile, you have to deal with all the other ******** of life and all the ******** you were keeping locked inside while using and all the ******** that you did to yourself and others that you now feel guilty about. Your trajectory in life evens out... addicts all start out so low and there's nowhere to go but up, but after awhile it's not as exciting any more
2. Boredom kicks in. I've been able to pour myself into school and focus on that, but with recovery it was different. The results were so immediate, until they started to become just a fact of life; that I was sober. So there was less to be excited for and we all know that doing drugs made boring stuff more fun, so many of those things you loved to do (or people you loved to hang out with) aren't so great without the drugs.
3. Loneliness. For me at least, relationships have been harder. You have some weird bond with using friends and being high on opiates made us feel close to each other a lot of times and we could just talk for hours. It's a little different now. And for me, it's harder to relate to "normal" members of the opposite sex now since I was so entrenched with girls who had used.
Any of that ring true to you? I've found ways to deal with it all. I've found some things to do with my time. I've been working on relationships with people who don't use. I've gotten into a couple old hobbies/interests I left behind when using. I've never been quite where you're at with feeling as down as you are, but I think it was important for me to recognize those things about myself and to understand the things that were bothering me; then at least TRY to fix them or find some answers within my negative emotions. When you're feeling badly, just sit down without distractions (maybe even with pen and paper) and try to really let the emotions wash over you and figure out what it is that is bothering you (either with general things in your life, problems or what you want to accomplish, or just basic feelings/emotions) and then try to find solutions to them.
I learned a lot in rehab and a lot in the few months I was doing NA. After awhile I felt I had learned all I could from them and it was time to start applying it to my life. You could try NA again or not, but either way I think it's important for you to figure out what recovery means to how and how you can apply it in your life. Like I said in point #1, life just doesn't instantly get better after you get sober... there's a lot of broken pieces still left that you alone have to pick up and figure out a new way to put them together. Best of luck.