In the pickup game we were playing last year, I was in the same spot you are: the rogue/fighter who was basically the only guy able to stand in the front line and protect the fragile spellcaster-types. It's not an easy gig, is it?
But honestly, you're doing a fantastic job at this already. I mean, an AC of 28 and the ability to deal out some actual damage even when you're not getting a sneak attack? You should be damn proud of your work so far. You're already using feints to get the sneak attack in even though no one's there to flank with you, and it sounds like you have a nice collection of items to back yourself up with. So far, you're doing great.
But I can see why the hit point situation would be becoming more and more of a problem as y'all gain levels; you're starting to hit the "glass ninja" stage of rogue/fighters, where your AC is high enough that the only things which can regularly hit you just happen to be the bad guys who can do enough damage to kill you in just a few rounds. And at that point, your melee career starts to look a bit tenuous.
So first of all, you really need to start consistently using buffing spells and/or items. Patryn of Elvenshae's absolutely right about getting yourself a miss chance: trust me, you'll be amazed at how much difference even a 20% miss chance will make in terms of your survivability. Get friendly spellcasters to buff you, get potions and items that let you buff yourself, and if you've got the Use Magic Device chops to use 'em, get scrolls or wands, too.
Step two is something that I'm sure you're already doing, but could probably stand to emphasize more: avoid fights whenever possible. The sneaky and social solutions to problems get you just as much experience as fighting would (assuming your GM doesn't have some kind of unnatural prejudice against nonviolent tactics), after all. Sure, you don't get to take their stuff, but at least you're alive. The bottom line is that some opponents will simply be too tough for you to ever beat in a straight-up melee slugfest, no matter what feats or items or spells you have at your disposal, so you can either get used to finding other ways to bypass those obstacles, or you can get comfortable with the idea of your PC getting killed. A lot. Don't fight unless it's basically unavoidable, and if the rest of the party complains, tell them they're free to step into your role as the two-round speedbump standing between the horrible monsters and the delicate back row any time they feel like it.
You might also try to find ways to make better use of the terrain. Sometimes it just takes the minor pressure of asking to get a GM to put fights somewhere other than on flat, open terrain; if you've got feats or items or skills that help you be more mobile than the bad guys, being able to actually use that mobility to your advantage can make a big difference.
I'm also a fan of two-weapon fighting for rogues; more attacks = more sneak attacks, and that's always a good thing. Of course, you'd have to forgo the use of a shield, but at worst that just means that you've got another combat option at your disposal, and that's hardly a bad thing.
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seriously, though, you're doing almost as well as any character can in that position
ryan