I watched it through about 3 times trying to be nit picky, since I think that's best. I have a lot of experience fighting Fox, my bro mains him. If it seems like I listed a lot of stuff, then don't be torn up about it, it's better that I be picky than to be too nice and say that everything was perfect. That certainly wouldn't help you as much. Also, how long have you been playing Sonic? I don't recall seeing you around before. If we haven't met, then nice to meet you. If we have, then oops, sorry!
0:05 - Move your up air to the left when he is shielding at the edge of the platform. The first hit slid him to the edge. The second would have knocked him off. He likely would have fallen off and failed to tech and given you a chance to punish him. This is a small thing, so it's not too big a deal, but sometimes small things can add up to give a big advantage.
0:10 - 0:21, as well as many times throughout the match - you were giving Fox too much space. He was just stepping back a bit more and firing off lasers. Stay in his face and create pressure. Your spins consistently were taking you so far through him that he had time to punish with lasers.
1:47 - Look at how soon you started your fsmash. Literally pause the vid if you have it up. He was still in hit stun when you began it. He saw the attack coming from a mile away because you committed to it too early and he had sooo much time to plan his response and you were stuck committed to the move.
1:49 - I'm not sure in this instance if it were possible, but by SDI-ing down and to the right while holding shield you might have gotten your shield out in time to block the up smash. Again, I'm not sure how new you are to Sonic or brawl in general, so if you need any explanation of SDI, just let us know.
2:15, 2:17 and several other times - use down smash sparingly, it lasts so long that it gives your opponent a ton of time to prepare an attack if you miss.
2:30 - you had a good thought to move back to punish the illusion that you read coming. its great that you predicted his move. however, you stepped back a bit too early and he read you in return and did his illusion to the platform forcing you to miss.
2:34 - generally starting a spin dash that close to an opponent can be dangerous if they aren't too scared to try to attack first.
2:45 - once you see that the fsmash is going to miss, just release it. holding it only gives him more time to plan his punishment.
2:55 and 2:58 - spring chasing to up air can be useful, but if you do it every time, then the opponent learns to avoid it. mix things up and do spring > bair or fair sometimes and you'll find that not only will you catch them by surprise with those attacks, but that it will make them have to fear those as well and sometimes trick them into landing into that upair as they try to avoid a bair or fair. think of it like rock paper scissors. you can't always throw rock, you have to throw scissors and paper too sometimes.
4:44 - try to avoid letting fox get underneath you if he is on the ground(especially at high %), it tends to lead to things like that. also, if he hits you into the air such as at 6:15, DI away and not towards.
7:05 and the match overall - spin less and use running mind games more. Though it IS good that you used the stages slopes for iSDR.
Also, try approaching with spin shot. i don't think i saw you use it offensively even once. Also, use ASC more. You use the spindash a lot but hardly at all use spin charge. I saw that you are aware that ASC can cancel into a shield. Use that as a mind game. You can ASC > shield grab for example.
If you are pretty new to Sonic, then it's that you know some of his general tricky tactics. I saw that you are aware of shield canceling both the spin dash and the asc, that you know the spin shot, and the iSDR. You read your opponent well in some cases. You just need to work on making some better decisions and learning a few match up tricks. With a bit more knowledge and a bit more experience that Fox player would have been in the palm of your hand. Keep working hard and you're sure to see the improvements you want.