As the others have mentioned, the household income is taken into account when any member of that household applies for job seekers allowance. This includes what they describe as co-habiting so you and your girlfriend will fall into that category. What they will do is look at the total income that is coming into the house (eg, wages) and other things such as savings. I also think that you are experiencing problems with your girlfriend’s income being taken into account because you haven’t worked over the last year and have had to apply for income based job seekers allowance.
I know it seems unfair but I’m not sure that there’s an answer other than, as Loztiggy suggests, checking with Citizens Advice that you are being treated correctly. They will go through all of the information with you to ensure that you know what you can and can’t claim.