I haven't played Shyvana top a lot, but I do have a lot of experience with her in the jungle. I think the honest truth is just that Shyvana top is bad and hasn't been good for a long time. In fact, I think Shyvana is pretty weak overall; she was nerfed a bunch to make way for devourer and then devourer got removed and she wasn't compensated for its removal at all. Perhaps it's due to 
inheriting sated devourer's passive, but Rageblade isn't really worth it anymore.
As for the matchups, Nasus' W (wither) reduces attack speed which Shyvana relies upon. Shyvana has to be in melee range to do damage so Nasus can kite her easily early game and then beat her late when he's stacked and can drain all of her attack speed. Nasus E (spirit fire) also reduces armor, so that kind of nullifies Shyvana's passive. So yeah, I think it's a pretty bad match up for Shyvana. Best bet would be to get a jungler to gank him early. Try to dominate the lane early; if you get a lead on him, it should be near impossible for him to stack his q.
Mini Gnar is ranged, does % health damage, gets a movement speed buff every three hits and has a slow and a jump, so he can kite Shyvana pretty easily, and if Shyvana gets kited she can't do damage. Mega Gnar gets so many stats and has so much cc it's pretty hard to duel with him in the first place, so yeah, Gnar probably wins this matchup too. Best bet is probably to try to farm up as best as you can and build some armor.
Maokai is pretty strong right now, but I imagine if you play it right you should be able to beat him early. He might build frozen heart which is a good item against Shyvana, but something like a 
or even maybe a 
would be nice to help you cut through him. After your first damage item you probably would want to pick up some magic resist and then build tanky for the rest of the enemy team.
Pantheon beats pretty much everyone early, and his passive is really effective against Shyvana. I think your best bet would be to try and farm and get an early 
. You can probably start to beat him around level 9 or so, as long as his lead is not too large. Pantheon is tough to lane against as anyone though and can snowball pretty hard.
I can't see your match history unfortunately (maybe because we're in different regions?) and I don't have a ton of experience in playing Shyvana top, but I have lot of experience in top in general and with Shyvana Jungle, so hopefully I helped out a bit.
