If you are trying for a second baby that you don't want, you should take a breath and stop trying while you sort out your feelings. I was in a similar situation - Clomid, getting older, etc. I was eventually successful and had a beautiful baby boy. He is now 19! We weren't sure about a second child, and we went back and forth a lot.
While I can't tell you whether you should go ahead or stop for good, what I can tell you for sure is that you do NOT owe your child a sibling! Do not listen to anyone who tells you that single children are lonely. In fact, I don't use the term "only child" because it sounds like that child is deprived - he isn't! He's a singleton. What you child might miss in "company" he gains in independence. He might not have sibling companionship, but he also avoids sibling rivalry. He doesn't need a sibling to learn to share - that comes in preschool and playgroups and life experience. My son has learned to find and make friends in all kinds of settings - he made friends in the neighborhood and through playgroups (we found some through the Newcomers Club but there are lots of other ways). When we went on vacation, he made friends with other kids on the beach. He learned to interact with all kinds of other people, and has always been a social kid. He developed confidence, leadership skills, and an outgoing personality. At the beach, he would take a clear plexiglass container and collect crabs and fish and other items, like most kids do - but he developed such an elaborate "habitat" that other kids gravitated to him - friends came to him, is what I'm saying. He would build castles and dig trenches to the water's edge, and soon other kids would join in, and he sat there like the master engineer, directing operations and getting other kids to gleefully join in.
He is not selfish or deviant or lacking in anything. We hiked in the woods and caught frogs/toads, inspected lichens on rocks and fed carrots to the neighborhood horse. We took him to museums and enrolled him in playground programs, preschool, soccer, etc. And not a million things at one time either - when he was in elementary school, he went to religious school and was allowed to have one other activity (sometimes it was soccer, once it was basketball - not an activity every day!). We said "no" to him even if we didn't have another kid for him to share with - he didn't get everything he wanted. When he was about 9, he started helping neighbors out by bringing in their mail when they were away, putting out their trash, walking their dog or feeding their fish - whatever. He grew into bigger jobs, mowing lawns and spreading bark mulch. He started his own little entreprenurial business, built a customer base, and even learned how to do invoicing. He was recruited by more than a few colleges because of his independence and initiative. He developed into a terrific track star with the help of a great coach who saw his independent spirit and maturity, and he became a team captain because of his ability to lead. All those years digging in the beach sand turned into an ability to lead 125 guys on a track team!
He's not a miracle - just a great kid who made the best use of his particular situation. With the right guidance from 2 older and experienced parents, your son can do the same thing. I doubted myself a few times but I can say with the wisdom of hindsight now that we absolutely did the right thing for us. Families come in all shapes and sizes. You do not have an obligation to put yourself through further medical experiences to satisfy someone else's idea of what a "perfect family" is. Having one child does not mean you are a failure in anyway.
If anyone said to me, "You have JUST the one child?" I said, "He fills my life with love and joy." If they said, "But won't he be lonely without a little brother or sister?" I said, "No I don't think so at all. He has lots of loving people in his life." If you say it often enough, you will convince yourself.
It's a lot like people who have no children - others ask if they are selfish or infertile. Nobody's business, and it might not be either!!! If you have a backbone, and if you have confidence and self-assurance and a real purpose in your life, you will come to see that any family size, arrangment, lifestyle and values can work if you believe in yourself.
And the people who say you need 2 kids have also heard from others that they should have 3, and those with 3 have heard "Why not have 4?" What about those with 3 boys who have been asked "Are you going to try for a girl?" Let's all treat each other with a little more kindness, and not put our doubts on others. Mostly, people who say you need a 2nd child are trying to justify their own decision to have 2, or are not able to see that what's right for them might not be right for you. For every decision, there are advantages and disadvantages. People with 10 kids find that their kids have lots of playmates and also that they have lots of babysitters, but it's also hard to find an activity that everyone can enjoy together, right? So it's really about finding the right balance for YOU!
Stopping with one child doesn't mean you don't have enough love in you for two, or that you are selfish, or that you are incapable. It means that one is right for you.
Good luck!