I've been there and made it out. You're acknowledging you have a problem which is the most important step. Once you accept that you want to change -- deep down -- you've got the seed for change. Here's what I would do to make it grow:
Do you exercise? Even if you don't, do something that will just leave you completely physically exhausted. Run as far as you can, play a competitive game, ride a bike until you can't anymore. Just make sure you're so tired you fall asleep quickly tonight. You don't have to do this every day, but you need to go all out tonight. You're going to give your body a reset and mind a reboot this way.
The next day when you wake up remember that you made this decision the day before, and you have a whole new life in front of you. Say to yourself, "fuck yeah, this is awesome, I'm going to take control of my life again. I'm gonna be the person I know I'm meant to be."
After that, when it comes to work, set short goals and stick to them. Stay off social media. It could be "I'm working on this feature for an hour", and get it done. At minimum, try and get 1 thing done before lunch and one thing done after lunch.
When you get home for the day, I recommend another healthy dose of exercise just to get you to sleep again. After that it's basically wash, rinse, repeat except you start adding slightly bigger challenges and better habits into your life each day. Start adding healthier diet choices in, etc... Make sure you have a non-work hobby for pleasure to look forward to after work. If you really have to smoke (no judgment sometimes we need the escape from the rat race) then do it right after work (not before) and do some yoga or stretch or something instead of tv or video games. You'll feel rested, your body will exert some more energy, and you'll sleep better than if you smoked right before bed and you'll work better than if you smoked before work. As success piles up you'll lose the desire to smoke and you'll realize you don't need it.
The important thing to remember is you don't need to change everything at once, but if you need to break the cycle I've found that an intense physical use of the body is usually the best starting point for a reboot at any time in life.
You can change, you will change, you're still super young. I was in a similar boat at your age (31 now) and the post-college, starting workforce blues are a real thing. Took me some practice to get over it but it's worth it. Best of luck. Also this kind of post might be better for a reddit forum than HN, although generally the HN crowd gives better advice. :-)