Hello, I feel your pain on this. I have a DD who is about to turn 4 in October and we have been trying for a sibling for almost three years, not even whisps of bfps.
We've had one unsuccessful round of IVF and just had another I'm in the tww for, but if that's unsuccessful I'd have to do yet another retrieval.
It took us a year to conceive our first so I was not entirely surprised when it started taking long for our second, but I had hoped maybe the first time around it was just down to bad luck. So I have the experience of primary and secondary infertility both: and honestly, the secondary sucks but the primary was so much worse, I cannot be afraid of never being a mother anymore and that was so hard.
We have not discovered a clear reason. I'm 32 and husbands sperm is ok and we do get decent mature eggs to fertilised ratios and even not-horrible fert to embryo ratios, but I don't make too many eggs and I seem to have a high number of immature eggs each retrieval so far.
One plausible reason I have is luteal phase defect, because mine is only 10 days, despite good lining and normal progesterone levels when I've been tested (and we did try progesterone supported cycles two years ago and it did nothing). When we had DD I got a bfp 8dpo, even evening of 7dpo, so I'm thinking I need to have a fast developing embryo that implants fast to even have a chance because I already start spotting on 9dpo and the slow to get there ones simply won't have anything left to stick to.
It's kind of bittersweet that my kids will at best be 4.5 years apart. How much will they really play together when they grow? But also, I honestly am not sure anymore I'd be cut out for 2 close in age when they're toddlers, seeing some of my friends and acquaintances struggle with that while I've had some room to breathe and do me things for a few years. I also have a trio of sibling relatives with 8 years between each and even though they didn't play together as kids, they've always been close and now as adults, thick as thieves.