It sounds like to me that you ovulate. Although, it is true that a pos. OPK doesn't actually mean that ovulation is taking place. The pos. OPK is your LH surge, which happens about 12 to 24 hours before ovulation. But, since you accidently got pg. with your 1st and since you are regular, it is likely that you do ovulate. I know that six months seems like a really long time but it really isn't that long when you think about the percentages of a healthy couple being able to conceive any given month. It is only around 20 to 25% in the first 12 months and then goes down if a couple hasn't conceived in the first 12 months.
I would give it a few more months and then seek some help from obgyn and then if that didn't work, seek out an RE.
Good luck to you!! I hope you get your BFP really soon. By the way, my DD is almost 3, too. I think you said your child was close to 3. I know how badly you must want to give your child a sibling. I want the same. My DD prays every night and asks God for a baby sister.
Anna Leigh