You know. I'd really hate to lead someone to this, but if you're realling having problems, why don't you just turn on 'Tour Invulnerable'?
I never really had any trouble with any of the missions. THe only one I found really screwed up was the first real Rebel mission..'cause something in the UCP kept destroying my damn ATR as soon as it left hyperspace. So I just had to make it invincable and I was good.
The game just takes patience. Lucas Arts and TG did a good job representing the SW universe, and made it realistic and canon as they could without making it impossible. It's not like most shooters or console games where a single X-Wing can take an ISD down without taking a hit..It just can't happen.
Another thing you may want to study up on is squadron tactics. For example. By targeting something, and pressing Shift-A, your squadron members will assist you in attacking said target. XWA came with a nice hotkey sheet..Just study it. And be patient.