Hey welcome!
You came to the right place. GYE is a community of people working together to help each other beat the YH. Sounds like you'll fit right in
It sounds like you've made tremendous progress on your own. That is truly impressive and you should pat yourself on the back for having come so far.
Stick around and check out some of the excellent resources on the site. See the Flight to Freedom program to help you learn about your struggle and some powerful tools to help you fight, and the excellent book the Battle of the Generation to help you reframe the struggle as an opportunity. (Links are in my signature.) The most powerful resource is of course the chevra. Connect to friends, find a mentor, and keep on fighting.
The 90 day chart has official rules - I'm pretty sure looking at Youtube thumbnails for a second doesn't violate them. That being said, many guys have found Youtube to be a very slippery slope and that cutting out youtube was a huge help.
I relate to your work computer issue very strongly. My work computer was a huge issue for me until one day, with endless support from the guys here, I put on my big boy pants and asked my boss for permission to install a filter. Guess what happened?
I got fired.
That's a joke. I didn't get fired, it was no big deal, and I installed the filter. The filter has been a huge help. Putting on my big boy pants and asking was also important (for me, at that stage. Happy to schmooze about that more if you'd like.) In short, in might be more possible to put on the filter than you think.
Until that point, there are browser extensions that are helpful. Wizimage that will block all images, and there are various site blocking extensions you can use to block specific sites. Because they work on the browser, not the computer, and are so easy to remove, they are more just temporary stopgaps to help you in the short term (though Wizimage is very helpful in general).
Although filters are a very important tool, it's doable to get clean with consistent use of unfiltered internet. That's because the change works on you, not on the computer. But why make life harder for yourself?
Hatzlacha and keep on trucking!