20 year plus pothead here as well. Just want to let you know that it definately can be done. I was pretty much a 24/7 smoker for at least the last 12 or so years and still smoked heavily the years before that. It has caused me much heartache and grief (most of the gory details are posted elsewhere and on my blog here).
About 8 months ago, I finally had enough and just quit. I had "quit" hundereds of times before, only to pick up again, days or weeks later. I honestly don't know what has made this time different. Part of it is that I have moved and am no longer around the same people, but it's not like I couldn't find it here if I really wanted to, I just don't want to.
The best advice I can give you is to, and I know this sounds cliche, "take it one day at a time." Don't worry about tomorrow. I don't even count the days. I have a rough idea, but really the only day that matters is today. As long as I can manage to stay clean "today" everything will be ok.
One of our members here calls it his 2 step plan:
1) Don't smoke dope today.
2) Tomorrow, repeat step 1.
Sounds simple, but is remarkably effective!!! You will probably find yourself to be a bit edgy for the next few weeks or so, might have some issues with sleeping, a bit of a loss of appetite to start with, mood swings and some depression are common. These things will all go away, you just have to work through them. It may help to talk to your doctor, get a check up, and tell him what is going on. Pot is primarly psychologically addictive, but that doesn't make it any easier to quit than any other drug, in fact sometimes it can make it harder, as it is easier to convince yourself that it's "harmless". For some people, maybe it is, but for folks like us, don't believe it!!
There are alot of recovering potheads here at SR. You don't necessarily hear as much from them as others, or they often don't talk about their DOC specifically, but trust me, there are many of us here and out there as well. If you need any support, feel free to post here, or shoot me a PM (personal message, you have to post at least 5 times before you can access this feature). There is a ton of support here, you don't have to be alone with this. Take care.