The other issues I have seen with the Rogue is the sheer amount of AP it requires to get behind an enemey, sneak, then backstab is crazy, not to mention if you can't sneak because other enemies are facing you(And see you) you have to use invisibility which is another 3 AP before you can even go into sneak mode.
If you're spending all your AP getting into position, you're doing it wrong, and something is off with either your build or playstyle.
If possible, have the Rogue stay out of the encounter, and when the rest of the party triggers it, move the Rogue around the back in sneak mode. You can then start the fight in position with no AP cost.
The Rogue starts with Tactical Retreat, which lets you move 14m for less AP than it costs to walk there. That helps. Points into Speed also increase your movement a lot, which is key for the Rogue. Try to wear armour with +Movement.
If sneaking is impossible, then it's impossible and you'll have to make do with normal backstabs, which are still worthwhile. I find invisibility's best uses are to either get all the way behind all the enemies, or to escape if you are surrounded. Using it to get into the middle of a pack for a single sneak-attack backstab, with enemies on all sides is probably not a good idea.
Also there is many times I'm behind a monster, in sneak mode, and execute the attack but it says sneak failed/ back stab failed? Not sure why this is, but when it does happem the damage is so low.
Sneak failed means you have left sneak mode. This is okay and you still get sneak attack damage. If you're not getting higher damage or backstab/backstab failed, then you are in the wrong position. Backstab failed means you were in the right position for a stab, but it failed. Those happenn.
The last problem I have seen with a rogue / shadow blade is you have to be SO careful with your party combo's as if you have a party member setting the ground on fire, electrifying it, poisoning it ect, you can't get in back-stab places without taking a bunch of damage which to a squishy class is daunting.
Tactical Retreat. And you have to co-ordinate your party so you don't do that.
This was one other thing I wasn't sure how its working. The talent that gives you +2 ability points, how does this work? I have had it one of my chars for at least 3-4 levels and haven't seen the extra points?
It gives you those immediately/after character creation. It is NOT +2 ability points every level, wow that would be broken.
