My FET cycle, I started out with a very low beta for the time they tested (30 or 33, can't remember now!)...the second beta did not quite double and the nasty weekend IVF nurse told me that I should just not be that hopeful but the RE wanted me to stay on meds and test again...tested again and it had gone up enough (more than doubled from 2nd number) that they said I should keep going with meds and be "cautiously optimistic" but also prepared for anything. By that point, I knew that it could be blighted ovum, ectopic, etc and asked my actual IVF nurse that and she agreed but said I would probably start having symptoms or my betas would start doing weird things...instead they kept doubling but always at the very LOW end of where they should be. I also spotted the entire time which I had done with my successful IVF but still scared me even more given the less than stellar betas (since my first pregnancy did not have beta issues)....anyways, as you can see in my signature and picture, I was a lucky one that had it work out well. According to my RE, frozen embryos often start out with lower betas because they are still hydrating when they place them so it takes them longer to implant. And some fresh embryos can be late implanters as well.
The reality was that at the early stage, it was 50/50 for me as well. I was fully prepared for it to not go well and didn't expect the surprise relief when 1st ultrasound showed heartbeat. I even had a frightening ER trip after that 1st ultrasound after a super heavy bleed that I was convinced was embryo miscarrying and was probably because of low betas at beginning...it wasn't, it was just a bleed and baby was fine and once again we were VERY lucky.
I want to send all the positive prayers and thoughts that your 50/50 shot will end up successful like mine. But since we were prepared for worst, we were already formulating our next steps if it did end up negative. I hope you will do the same if the outcome is not what you were hoping.