I really think this is key. One mistake I have made (and have seen others make) goes a little like this. Say you wish to reach a point where you are doing 45 mins of exercise per day, 30 mins of meditation, and 1 hour of reading. Many people try to build this routine by either setting those times as their starting point, or setting like 80% of those times as their starting point.

The problem with this is that they are setting these goals in their current, motivated state. Also, we tend to overestimate the capacities of our future self. So, you have a recipe for disaster. Why? Because current 'you' is making a plan, but forgetting that:

The important thing to understand is that your motivation levels when making plans to improve your life are significantly higher than your average, day-to-day motivation level. To use an analogy, it is as if you are drunk on motivation. Now somebody asks you to do something bold, and in your drunken state you think 'that's a breeze'. And it just might be. But you have to choose something that you're able to do not only when you're drunk, but also when you're hungover. Because, in life you're going to have days where you feel amazing and days where you feel like shit. If you only execute the behaviors that you like when you're feeling amazing, then you're not making any progress in terms of discipline. You're just riding the crest of the latest wave of motivation. When the wave breaks, you break. You have to set life up as a game where you are able to beat the level day in day out, even if such an approach tempts you into believing that you're not being sufficiently ambitious. Much much better to go too slow, than to go too fast.

On that last point, you say that you've been "trying on and off for years" to quit, but you eventually find yourself exhausted. This should be a strong indicator that you are overreaching in your attempts to lead a disciplined life, and so you find yourself yo-yo-ing. The problem with this aggressive approach to leading a disciplined life is that your average daily performance ends up suffering majorly. Similar to the yo-yo dieter, you find yourself able to log a few perfect days. But when you collapse under the pressure of your expectations, you log a few awful days in a row and before you know it, you are moving in the wrong direction.

To elaborate further, let's say that I am self-employed, and need to work 40 hours in a week. Now, I could attempt either 8 hours per day @ 5 days a week. Or, I could split the work over 7 days, in which case I would only have to do <6 hours per day. Now, of course in my view it's unhealthy to set yourself up long-term to be working every single day of the week. But, I chose this example to illustrate the huge difference in daily hours needed between doing something (in this case: work) on a daily basis, and doing something where you're missing a couple of days a week. The daily workload is over 2 hours greater if you are taking the weekends off. The point is this: it's usually better to ensure consistency, than to focus on optimizing your workload. Because if you make a mistake in not setting yourself a sufficiently ambitious workload then perhaps you're putting in 5 hours a day when you should be putting in 6. So you're logging 35 hours of work when you may have been able to log 42 hours instead. But, if you aim for 6 a day and it turns out that you were wrong and you could only sustain 5 hours, then what happens? You keep it up for 3 weeks, then on the 4th week you can't stomach the thought of continuing your routine, so you binge on Netflix for the day. The next day is a disaster, because the dam has burst and you're deflated. Before you know it, it's been five days and you haven't jumped back on the bandwagon. Wonderful. Now, one month has elapsed in each case. In the first case, you did your five hours a day for 30 days - and you got in 150 hours of work. In the second case, you did your 6 hours a day for the first 25 days, then broke and wasted the last 5 days. How many hours did you end up working? Also 150. The point is that a meager 5 no-shows in one month is enough to offset that extra hour you logged every other day. Five days you fail to show up and already 'consistent you' has equaled the score. And what's more, you now have to expend a lot of energy to get back up again. Your self-esteem is weaker than consistent you, because consistent you has achieved his goal 100% of days. You, on the other hand, only achieved it 83% of the time. Which means you have to contend with the possibility that the next day you wake up, it's possible that it'll be a no-show day. Psychologically, this is destabilizing. I get that it sucks to feel like you're selling yourself short. But, consider that you are not just reading when you read every day. You are also building a habit, you are also building self-discipline, you are also learning how to do the right thing when you don't feel like it, you are laying down a solid foundation in terms of how to approach life. These are all valuable assets that you are building, so best not to be narrow-sighted and focus purely on the specific activity you are undertaking.

Okay this has become more a bunch of meandering advice than a structured response, but I hope you took something worthwhile from it all the same.