That's been my story for years. I hadn't ever seen it work for anyone else, so what made me think I could do it? I don't know why, but I think we all tend to overestimate ourselves in this area. We somehow assume we are capable of changing everything at once (or it's the opposite and we don't even try). Fortunately, through several people wiser than me, I was able to learn a new way of going about it. Give it a try if what you're doing hasn't been working.
1. make a list. Write down all the habits you want to start or stop, in order of difficulty. Having a list has helped me keep track of my progress, as well as motivate me. I have my list on accompl.sh, but paper works well too. I just prefer online because it gives me accountability(anyone can read my list), and is easier to keep track of.
2. create a trigger. If you're trying to start a new habit, a trigger is your reminder. When I started flossing, my trigger was touching my toothbrush. Every time I reached for my toothbrush, I would remember to grab the floss first. If you're trying to stop a bad habit, the trigger is its replacement. When I'm tackling one of those, my trigger is deep breaths or clenching my fist. You can try one of those, or pick something else that's relaxing or enjoyable. It's also important to be very mindful when an urge hits you. Take note of it, and let it pass.
3. start small. Most people fail, simply because they try to tackle multiple things at once or begin with the hardest habit. Doing that will only frustrate and overwhelm you. Look at your list and start with the easiest thing, or the one you are most excited about. Assign a month to each habit on the list, and be careful about getting ahead of yourself. :)