What level are you in the fight against Benezia? That's likely the hardest fight in the game, and people who do that world first without doing some of the uncharted worlds to gain a few levels have run into a world of hurt. (The same can be said for the krogan boss at the end of another major plot world.)
In vehicles, other people have had good suggestions. I pick one of two strategies with the Mako in a given fight. I either hang way back and use the cannon on zoom (jumping over incoming attacks), or I just drive hard right into their midst, hit the cannon a couple of times, and then get out of my vehicle and fight the (weakened, from my cannons) bad guys on foot. You get more experience that way, and you can use your abilities. Throwing a geth armature into stasis, or using AI Hacking on it, is all kinds of wonderful.
Also, just to cover all the bases for things that I've seen people miss:
- Are you upgrading your armor and weapons with weapon mods?
- Are you spending your talent points at each new level?
- Are you doing the same for both followers?
- Are you using your powers in fights?
(I've seen people not do all of these things.)