In the future, I will treat large blocks of free time as being a potential risk, and make sure to fill them up with productive activities.

A few years ago I wasted three continuous months of completely free time just by reading internet. These days it's like: "oh, I wish I had a week just for myself and my plans". But yeah, I am afraid of what a lot of free time could possibly do again. Which is why I wish only for one week, not three months. Problem is, it is technically much easier to get three months (quit the job, find a new one three months later) than one additional week of vacation.

instead I do the easiest, laziest thing instead, which usually means browsing the internet.

The important thing is to realize that you cannot solve this problem without learning to avoid internet. So make that your #1 priority, and focus on it fully.

I tried something similar recently, and now I only read internet on weekends. Even that feels like too much; I should try to make it just one day of the weekend. (Previous diaries: here and here, 5 weeks already.)

swearing every night that I would totally be a different person, berating myself when I failed, using physical impediments to prevent myself from failing (...) keeping a journal in which I listed (...) why goals were not being met

The thing that all these strategies have in common is that they are attempting to promote the positive emotion.

Are you serious? What exactly is the positive emotion of berating yourself? If you really mean it, I guess you simply have no idea about what a positive emotion feels like.

one concept that stood out to me was this idea that happiness doesn't exist; there is simply the absence of negative emotions

Also, chocolate cakes don't exist; there is simply the absence of non-chocolate non-cakes. And it feels really good, trust me.

There is some half-truth in that saying, and it is this: positive and negative emotions don't mix well. If you win a lotery and break your leg at the same time, the pain from the broken leg will probably consume all your attention. Negative emotions can push away the positive ones. Therefore, to be able to focus on the positive emotions, you first have to get rid of the negative ones.

It's like if you want to hear the birds singing, first you have to turn off your radio. But that doesn't mean the singing of the birds is merely the absence of the radio.

When I detect negative thoughts, I will focus all my attention on the sensations I am experiencing present moment, starving the negative emotions of the brainpower they need. (...) The way to prevent your mind from thinking unwanted thoughts is to focus your mind so intently on another, different thought that there is no space left for the first thought.

That feels to me like instead of turning off the radio, you turn on the TV even louder. And then you don't hear the radio. (But you also don't hear the birds. But that's okay, because they don't really exist; they are just the absence of the radio.)

Again, there is some good part in this: Not focusing on the negative thoughts, not repeating them... that's good. If it helps to reduce their frequency, do it. I just feel there should also be some cognitive component in this, like: understanding why those negative thoughts are wrong, or at least useless.