I think the key will probably be filling your time with other things and distracting yourself when your feelings are intense to give them a chance to ease. It's not just breaking the habit of turning to him when your feelings are difficult but replacing that with different coping strategies.
Are you saying you've decided to cut this contact now? Because that does have to be the first step, I don't think trying to do it gradually will work and as long as you keep seeing him you're reinforcing all the feelings keeping you tied to him.
Would you find a diary to record your progress and write down positives (however small) from each day helpful? It might be good for you to have written evidence that you are managing. "Today I got out of bed even though it felt impossible" is a positive, as is "I turned my phone off instead of contacting him" or whatever.
What distractions do you have at the moment? Especially when you're feeling low/upset/vulnerable?
Little craft projects can be good because they make you feel you've achieved something. Trying a new recipe each week or just compiling recipes to try. Researching a topic online. There are free apps for crosswords, sudoku, wordsearches, colouring... Virtual pet apps (some more irritating than others!)
It's not about viewing the distractions as how your life will be forever but having a list of lots of different short activities you can do to break your day up and then picking which ones to do in difficult times. Once you've broken out of this trap he has you in then you'll progress to bring able to fill your time with other things and friendships.
Maybe you could write a letter to yourself reminding yourself why you need to break free of him, how doing so will benefit you, and some of the hopes for your future you'll be able to one day pursue if you can finally break free of him? Plus encouragement and kindness to yourself recognising how difficult this is but that you're doing a brave thing, and this is what you can do to help... .
You've done really well to get to this point, so I really believe you can take this next step.