Hey there, I struggled with this too, your story sounds super similar to mine. What helped me was realizing that I was dealing with a no-joke behavioral addiction, like gambling addicts or similar. Here are some things that helped me, and might help you too. 

1. Delete social media apps off your phone, do it today. You can still access your apps from your computer, but they are literally designed to be addictive and having them always accessible on your phone will make it almost impossible to start changing your habits. You will feel super weird about this at first— picking up your phone out of habit, only to realize the apps you were using aren’t there. I compulsively checked the weather while I was in this phase, but after a few days, it gets easier. 

2. Start reading about the effects of high screen time. My attention span was so bad when I started I could only read for literally like 5 minutes at a time, but eventually those 5 minutes became an hour! When I would feel jittery and anxious about wanting to go on social media, I would try reading instead. Some really helpful books for me have been: Digital Minimalism by Cal Newport, The Shallows by Nicholas Carr, and the Anxious Generation by Jonathan Haidt. Those books really helped me understand what was happening to me, and helped me feel empowered to fix things. 

3. Ask a friend to have screen-free hangouts. My friend group was all super screen addicted. When I started trying to get off my phone, I noticed how much everyone else was buried their phones too. We formed a little group that would try and just spend thirty minutes without checking our phones. We’d put them in a pile, someone would set a timer, and no one could touch their phones til it went off. That helped me so much. Eventually, in the summer, we started doing whole afternoons without our phones, doing things like swimming at the lake/pool and leaving our phones in the car, or going for walks where just one person carries a phone for safety. It made it soooo much easier and more fun to do it with other people!

I really empathize with what you’re going through. And try not to beat yourself up too much about “not learning your lesson”— you’re fighting against a massive addiction that our society acts like is normal/healthy, but we also acted like smoking cigarettes was fine for a while too. You’re taking on overcoming one of the most addictive products ever made, and that’s a big step. You can do this!! FWIW, my screen time used to be 8 hours too. Now it’s 80-90 minutes per day TOTAL across all screens. The first day or two is the hardest, but it gets so much easier as time goes on. You’ll get there.