I've been trying now 16 months and like you - I have this, every, single, Month.
In total I have had 15 pregnancies with only 5 being successful and carrying to term (one of which was a twin pregnancy).
Given the amount of pregnancies my body has been through, I put my lines down to residual HCG in my system. I had a blood test and my result was <2. Many women's HCG test (negative) is 0, maybe just over but not by much.
The FRER claim to be around 12.5miu/ml sensitive but have been tested to be as little as 6-7miu/ml - which is still triple what my residual HCG is. Therefore, it would be impossible to have a faint line, based on only 2miu/ml HCG.
So, if my beta is <2 there is NO WAY my lines can be caused by residual HCG still in my body from previous pregnancies. I personally think these FRER are not all they're cracked up to be. It is far too easy for light to get into the back and show up the indent / anti-body strip. When I take my photographs of HPT's I always have my finger at the back of the test to stop a faint looking line appearing and driving me crazy with line-eye.
I have tried many different brands of tests and unfortunately, I think the only thing that can be done is to, not squint that hard. If it's there, you'll see it. You shouldn't have to tweak the hell out of a
or squint that hard it gives you a headache.
I also recommend using several different brands at once, that way if you think you see something on one test, you may get a clearer, more visible line on another brand.
I agree it's a good idea to check your HCG level via blood test (beta), but I highly doubt it is caused by residual HCG in your body and I've breastfed all my children and never heard about breastfeeding causing HCG in your system..