I am preparing for campus placements but the company's have already started to visit. Should I go for them or not?

You should not skip any of the opportunity that's coming your way. I'd say prepare as much as you can but do not miss the opportunity. The 'not done enough' feeling won't go away - no matter how much you prepare for any exam. 

The reason I'm advising this is because the job markets these days are very volatile and getting new opportunities is difficult. 

Instead, analyse your last interview and find out specific points that went wrong and see how can you improve them. It's quite possible that the next interview with the new recruiter would be very easy. 

Hope this helps. All the best!