car seat not fitting??!

We have a 2000 mercury cougar. The back seats are 'bucket' seats I guess you call it. They are very deep and when we install the carsest with and without the base, there's a huge gap between the seat and the carsest. The seat sways more than one inch side to side and it's freaking me out.
We had a van but had to quickly replace it due to unfortunate reasons and this couger was all that we could afford right now. So we didn't have this car seat issue before. I'm too scared to take LO anywhere because of the giant gap and swaying. I'm looking into going ahead with a convertible carsest but am unsure what would work best for this car...?