Hi and congratulations on your pregnancy.
I haven't had quite the same experience, but did once have hcg levels measured low at only 7, but was about 10dpo at the time, and did not get a positive home pregnancy test til two days later.
It is hard, but try to stay positive. Doubling times for hcg are an indicator of whether a pregnancy is going ok, but they are not definitive either way.
Low hcg and lack of good doubling times can lead to a perfectly healthy pregnancy (my mum and sister never ever got positive tests with one pregnancy each). Whilst I have had pregnancies with perfect hcg levels and doubling, for it to still end in miscarriage.
My advice to you is to try to take one day at a time, the mantra"today I am pregnant" is helpful here. If you feel you must test, the please do not use the conception indicator ones, they are expensive and not at all accurate as they depend on the concentration of urine etc. If you feel you need the reassurance of testing at home, use the cheap ebay or amazon HPTs. Then you can test more and remember that the tests will only gradually get darker and a one off day where it is lighter could just be urine concentration was greater the day before etc.
I hope that you get some reassurance soon, are you going back did another hcg quant test soon?