First, thanks for posting this video, it’s hysterical. Fireball, puddle, fireball, puddle, fireball, puddle. The funny thing is you had lifebar advantage and were at a basic stalemate for a while. You just lost your concentration and started getting hit.
Here are my thoughts - I am not being critical. I know these are frustrating matchups.
I don’t have high level matchup advice, but I can certainly comment on this video. During most of this match you are two inches away from being able to hit him and start a combo. But you let yourself get intimidated by being hit by fireballs in the air. Take that hit if you need to. The fireballs do only slightly more damage than if you block them. That’s why this match takes so long and ends in a timeout. If you have to take even 5-6 hits with a shago fireball to open him up into a real combo, then it is more than worth it. You are trading 10% damage for the opportunity to do a 60% combo with Glacius if you have meter. In the video, you jump - get hit by a fireball and then basically give up. You shouldn’t jump every time - make him guess what you are going to do. But you should definitely have been jumping in more. I believe (although I can’t prove it - so someone tell me if I’m wrong) that you can poke him out of slide with a light kick or mk timed correctly. So if you jump over his fireball expecting him to hit you with slide, throw out a LK or MK instead of trying to hit him at range with the HK.
Once you opened him up you didn’t maximize your combos, which is just something basic to work on. At about 20 seconds you hit him with a jump kick into cold shoulder opener and then just dropped the combo. You could have taken 45% off his life bar there and you settled for 11%. Those things happen, but you are going to lose to a lot more than spam if you don’t take advantage of opportunities like that. Not only that, you never tried this successful tactic again in the match. I gather from your previous responses that in the prior game he reacted pretty well to jump ins. But even so, it’s clear in this match that you have surrendered that option so he doesn’t have to worry about it. Sometimes it’s good to let him know that you will do even low return moves just so he has to consider them in his mind. And again, at least the one time you did this at 20 seconds, it worked.
I know counter breakers are always high risk/high reward, but if I see a player playing like this I would make him prove to me that he could consistently break combos before throwing out a counter attempt. Especially in a situation where you know that the match is a slow grind like this one. Pile on good, conservative damage when you can. A couple of combos and you would have run away with this one. Shago down by a life bar isn’t going to sit there for 85 seconds spamming fireballs while you puddle.
Never try to get into a straight up fireball shooting contest with hail vs Shago. That will only end badly. But as @Zcythe points out you might want to throw one up with lp, as a theoretical cover to jump through. The light hail could come down and knock him out of a combo if he catches you on a slide.
Remember that, although his fireball will knock you out of a combo with shatter, shatter as an opening hit does a LOT more damage than a shago fireball. People get disappointed that they are pulled out of combo and think they have “lost,” but I’ve actually won rounds against people who thought they were beating me by knocking me out of my shatter combo with their fireball. So I just kept doing it and they kept doing it and it didn’t take them too long to figure out that I was getting WAY ahead. Oops. It takes some timing to get one of these out against fast fireballs, but you did it here at least once.
Finally, you didn’t capitalize on your knockdowns. Even on soft knockdowns you should be trying for better positioning. At one point, you break his combo and then you jump away. You hit him with your j.HK so it looks like you did something smart, but you just lost the positioning battle. He will take that trade all day. You have lots of options to follow knockdowns. None are perfect, but many are better than retreating back to stalemate range.