What they're all saying. Variety in the weapons you choose in your fights will be your key to success. Not being able to dual wield as a felar actually simplifies things quite a bit, but you can be pretty effective if you take your opponents' weaknesses into account.
What worked well for me is to try and carry some light non-physical sword/dagger variations based on your enemies. It's tough to balance being light for dodge, and carrying extra weapons at the same time, but it can be done. If you can, sacrifice the temptation to load up on heavy metal gear in lieu of wearing light gear +stat gear. Because you're an assassin you should almost always be able to pick your weapon according to your opponent before the fight begins. Equipping for strength/saves/hitroll can be done really effectively with light gear and works pretty well for an assassin. Think of your vuln/weapon exploiting tactic as a surrogate for a high damroll.
Spear/staffs are great for general offense/defense balance. Start any regear with a decent light staff or spear (yew staff from eregion for instance) then take that and get some other weapons off mobs. Titanium spear for general mob fighting will allow you to tank almost indefinitely sometimes! Though- try and find a couple variations of light swords to switch up against specific enemies you have problems with as well as a light shield (light armor use will enhance your shield blocking!). It is a lot easier to find light vuln-specific swords than spears/staffs, so keep that in mind. Since you're CG- Thornblade (mithril) against drows is a good light one. Gleaming sword from Diana (wrath) is good too against duergars, and you can use it against giants because it's non physical. Flame blade or whatever it is against those evil felars. Then mix in some daggers if you can find some specific vuln-exploiting ones for later on assassinating goodness. Load up on all the gear from the felar chief if you can. It's not sexy in that you won't neceesarily have a high damroll, but the +STR, saves and hp will save your ass, allowing you to shrug off whirls, boneshatters and such.
One thing I learned the hard way is to always always always wear a shield/sword against dagger specs. Concealed's alone will eat you up because it fires off their dodge I believe. So you actually want less attacks and focus on a good vuln sword and kicks/kan/kot/owaza/blind/disarm until their dagger is gone. Then if you're lucky you can remove your shield later on in the fight and blast some surprising damage (assuming you are lucky and still hold ~your~ weapons).
Try different things! I still brain fart sometimes and, as Isildur said- check the class page real quick to remind myself what weapons my opponent doesn't use and switch up to that. Staffs for orcs/APs, spear for bards, sword and no shield for non-necro mages (w/ tigerclaw bonus!)... daggers actually seemed to work real well against orcs as wel because they tend to not practice up this skill much- use that light attack one if you can get your hands on it and you might be happy with the results.
Anyways just my two cents- assassin fights are often difficult to land kills with so you need to really plan ahead to get yourself the biggest advantage possible to quickly outmelee your opponent. I'm never good at sealing kills with an assassin but taking some of the above advice you should be able to run off most opponents and make you at least "feel" baddass ;).
That's more than I planned on writing, whoops.
