Hi ManWhoStumbles,
I am also a teen (I assume you are too) who is struggling with mzl and inappropriate images and videos that unfortunately is becoming an almost inherent part of modern popular culture. BH I dont have problem with p**n or n*dity anymore. Im 17 in 2 months.
I find my struggle similar to yours in that I too find myself wasting too much time doing unproductive things which may sometimes be chas veshalom sinning. There is hardly any interaction with people outside my family except in school (I'm in a public pre-college level school). After school I try to stay at home all the time except to go to the synagogue. It is only as I write this do I realised my problem.... The lack of social interaction is most probably because I'm lazy. I should try to change... Addiction makes us more isolated, I also feel that being isolated makes us more vulnerable to the attack of yetzer hara. I remember watching a video linked under "Markz"'s profile about the nature of addiction. Under isolated and high stress conditions our body tends to normalise unhealthy stimulation while with social interaction and healthy relationships our body rejects the unhealthy stimulus option. Not sure if I recalled correctly but the idea is like that.
Here is what helped me: I try to set myself a target, a goal that I wish to achieve (like getting rid of p**n/ stop watching youtube videos with untznius content). The goal must be something that you want to achieve in the sense that there are many pull factors (e.g. spiritual and physical health, a normal life). So far the reasons/motivations to fight your addiction, that you have underlined, are almost all push factors, they are about the harms the addiction is causing. Awareness of the harms of our addiction also helps me fight yetzer hara, and is perhaps the deciding factor that spurs us to start seriously fighting the addiction, and to sign up for GYE in the first place. But t is very important to have a clear picture what who you want to be, what kind of life you want to have. Only then can we (at for least me) start working on our problems with substantial progress. It is like you want to leave a place and it helps to have a destination in mind.
On the area of filling life with meaningful activities, I haven't had much success but from experience I have learnt that it helps to have a goal for the day, or even the next few hours. I always have a list of things to do in case I feel bored at home. Again I think having some objectives in mind in addition to the determination of breaking bad existing habits is helpful. Let's face it, it is extremely difficult to fill all our free time with the most meaningful and productive things (e.g. learning Torah or studying in general). It's good to find something light that you also enjoy (e.g. playing an instrument, reading a book).
I used to be addicted to games (at least 1.5 hours everyday and it was a battle royale violent type of game). Once you are in the game, it's hard to decide to close the entire thing, it's like another reality (a fake reality). BH I was lucky that I slowly got bored of the game I played and games in general. I was thinking that all games are pretty much the same, you control a character and you try to move your hands quickly. It dawned upon me that playing a game (we all want to win) is about seeing who spends more time practicing the hand movements, in other words to see who spends/wastes more time/a greater portion of life practicing the repetitive hand movements. This reality was in front of me. Winning doesn't have any meaning at all if you are not too concerned with who can do the hand movements faster. This may be insensitive to gamers but this is what helped me stay away from games and it is my genuine opinion.
I don't follow any sport but I think there is no harm with sports per se. Many people on GYE has pointed out that exercising helps in fighting s*xually-related addictions by acting as an healthy "outlet". So I think there is no problem with following a sport especially if you are looking at the technical skills of professional players and trying to enhance your own game. If you are in countries with football as some sort of "national sport", e.g. UK, it may be quite natural to follow the sport as everyone is talking about it. I once asked a (non-jewish) friend in school why he is always reading football news and watching football videos, he replied that for him it's mostly about the build up. I always see him reading about UK football which I feel is indeed over-advertised and even sensationalised. Of course I can't generalise but if you are following football mainly because of the speculation part or the sensation/hype (or even angst) then maybe stop watching football videos is a good idea. The general rule of thumb for me is to not follow anything that I have no stake in. I find many things especially in pop culture to be always seeking your attention and turning it into profit without any regard of whether you need it or whether the matter concerns you in anyway.
Last but not least, to me being overly ambitious in my plans to fight my addiction usually doesn't turn out to be very effective because it makes the struggle seem harder disincentivises me
Hatzlacha!