I still haven't figured out a way to beat him when he's already up in your face. You could use low punches, but he'll throw you faster than you can hit him. Perhaps you could throw escape by pressing LP and LK; I've never managed to master throw escaping in SFIV yet, though.
So part of it is luck, and another part of it is skill. Seth is HARD on any difficulty. Just try to avoid getting too close to the guy when you're fighting him.
In other news, I've only recently figured out that Ryu can hit anyone with an Ultra or a Super right after you use a Low Punch Dragon Punch on them. After you hit an opponent with a dragon punch, they'll start floating down. Once they're at the apex of their 'float' animation, you should have completed the motions for a Super Fireball or an Ultra Fireball. If you do it too late, your fireball will whiff and you'll be at a disadvantage. If you do it early enough, you will hit your opponent for some free, and unblockable damage! The earlier, the better....that's how it is with this super/ultra combo attack.
It's very useful if you've hit someone who is trying to jump at you and you have a full SUPER or ULTRA meter. It can bring you back in the game, or even win you the match much quicker than you expected. Good to save up that super meter for situations like that.