Seconding all the call outs about focusing on changing environment rather than relying on will power. In general, I think people have a tendency to expect to much on simply "willing" themselves into doing something, then feel guilty when it doesn't work, which erodes the willpower they stared out with -- recurse down to giving up.

The specific thing I'd add to the comments already here is four things that have worked for me. These are more about phones specifically than internet addiction in general, but I think they help with that larger topic too:

1. I bought a travel alarm clock. At night, I leave my phone downstairs. Having an alarm clock means I can no longer use the excuse "I just need the alarm function on my phone." This also helps in the morning as I literally can't just wake up and go online.

2. I removed Facebook from my phone. If I were on twitter, etc I'd remove those too. Communication is better from a keyboard anyway -- I'm on social media less frequently, and when I'm on I have an input device that actually supports writing full thoughts and sentences.

3. For web sites, I stay logged off on my phone. Similar to above, though less drastic than making those sites completely inaccessible on my phone. I still browse, say, Hacker News, but there's now often a forced wait between reading and replying.

4. I have a small phone (iPhone SE). It feels "cozy" (to one commentor's point about making the internet a more cozy place) since it actually fits nicely in my hand and pockets.

I guess I'll throw in another point too -- I experiment with different ways to change my environment. I think that's the real trick, more so than even the above -- I came to those through trial and error. Try changing something, and use the results of that to inform what you try next. Recurse to improvement. Good luck.