I'm with broken. I disagree that you can't compare grief either. You can feel very sorry for someone who has gone through the trauma of a transplant, but that doesn't really compare to the grief never finding the right donor would bring. I think primary and secondary infertility are very similar in that way.
It is very sad. However in this case one of the saddest things about it is that the OP seems to be missing out on the joy she could be having with her current child (taking him out and seeing him play with other children, socialising with him) because when doing these things she's dwelling on a sadness about something which may or may not happen in the future.
And yes, as someone who has faced the possibility of never being a parent it does make you very, very sad that someone is cutting off their nose to spite their face because they won't get to rexperience something that to some extent they're throwing away with the child they have.
Not to say the OP shouldn't grieve, but grief is something that can be dealt with and ultimately rationalised and certainly reduced, particularly with the right help.
I know people on this thread think they're being kind by supporting the OPs position without quibble. But really I wonder if someone lay on their deathbed what they would prefer their biggest regret to be? Not having a second child, or themselves and their child missing out on the happiness of that child's childhood because it was spent grieving for someone who never existed?
Yes there will be sadness involved, but it's absolute madness not to do what you can to mitigate it. Because then not only may you not have a second child, you will have missed out on so much happiness with the first.