I had the same issue when I first got my Scim. If you're coming from an escort or even a cruiser, the Scimitar will feel VERY off at first. It can't dog fight. Here is what you need.
-Borg 2 piece bonus. Preferably the borg Engines and Deflector.
-Plasmonic Leech console
- Run a Reman shield for it's high capacity to balance out the time between procs from your Valdore console. If you can't afford a valdore console, run elite fleet resilient shields with disruptor/plasma protection. If you don't have a plasmonic leech, run the maco shield with system power benefits.
The tactical Scim has 2 engineering consoles. I equip two mk11 fleet mine ultra rare RCS accelerators. (40% turn, 10 resist all, 10 kinetic resist each) It does much to curb the Scims weaknesses. If you can muster all this loot you'll be able to stick it out in a fight taking it while dishing it out. If you can't, then rely on the Scimitars battle cloak to get you from scuffle to scuffle. Time spent without your guns firing is lost dps, but it will help with turning at least.
1. Get a valdore and use the Shield Absorptive Frequesncy Generator on the scimitar. It's the single most powerful universal console in the game and only the romulans have it. Essentially, the more energy weapon damage you do, the more powerful the shield heals it gives.
2. The scimitar isn't quite maneuverable enough for a Dual (Heavy) cannon build to be the most effective, despite having 5 forward weapon slots. You can make it work very well with cannons, but beam arrays are the way to go with this thing if you want DPS since the turn rate is a tad low.
3. Use Aux to Bat setup.
Hull-wise, the Scimmie is actually pretty good. You might just be pulling extra aggro because of how much more dps you're inflicting.
I'm running Scimmies on two characters atm, one's a Tac AP Cannonbird, the other a Eng Rom-Plasma Beamboat.
With the cannon-build you can compensate for the time on target issue by just cutting back on your throttle and gliding to your target (start cutting back around 16 to 13km fom the target) so you can keep slow forward momentum going and still be able to haul/break off if needed. using that method, plus EptS/Sci-Team/Secondary Shields/Singularity Powers, the only time I die is when I do something stupid (which is admittedly somewhat often)
If you're trying to turret with cannons and torps, this could be why your defense is so low. Sitting in one place seems to be the lazy norm for most cannon ships. Stop doing this. You have a battle cloak, use it. Make your attack run, then make your escape and set up for another. There's a reason beam boats survive longer than cannon boats, and that's because they aren't sitting still trying to keep 5 guns on a target. This is not only bad play, its a bad idea. And your scimitar isn't exactly a tank. It's big, but that just means it has more target area. You have to keep strafing your target, otherwise concentrated fire on your forward shield will melt through it and kill you.
Despite its perceived opness. You can't just kit it and autowin.(except if you beam spam in it..then just press space bar with your eyes closed)
Seriously though..It takes some getting used to. The most essential bit of kit is 2 piece Borg. The other is using the shield while cloaked console. Third..valdore console. I know people say the valdore console is op but without it the ship is almost unviable. After that I recommend dikron doff and power on eptx doffs.
The reason it blows so easily is because of its size. The hitbox on it is huge so every time anything on the map blows..you take damage..for example when a cube blows you want to be around10k away!