We chose not to pursue anymore IVF after our two free NHS rounds.
Similar to another poster, I left my first marriage early-thirties (so grateful for no children with him!!). Met my now-husband at 35 and due to age we did start trying quite early on in our relationship. Nothing happened for two years, we received two free cycles and results from both were rubbish. (Our diagnosis was 'unexplained infertility').
I had spent hours on infertility boards on Mumsnet, and even had WhatsApp groups with people going through treatment, and all throughout it really struck me how indifferent I was to the process than everyone else. Other women seemed so terribly sad, and so desperate to have a child. It was all-consuming for them and their lives really did seem to be completely on hold until it happened.
For me, there was always a sense that I was going down the road because it was there, and because I should at least try, but I took huge pleasure in my life as it was. I am quite a maternal person but I have never felt that longing broodiness that so many do. For that I am very grateful as I witnessed it make so many people utterly miserable.
After the two rounds had failed we had to decide whether to pursue any more or not. That was definitely the worst part as the clinic encouraged us to, and well-meaning friends love to tell you about all the people they know who got pregnant on their tenth round or whatever. For us, we kept kicking the can down the road until suddenly it's 5 years later, we're early 40s, and clearly it's just never going to happen.
It's so much easier now that our friends are past the stage of exciting pregnancy announcements and all the territory that goes with it in your 30s. Now they are all knackered and stressed and have to plan a simple weekend outing 3 months in advance because of the kids' football/dance/custody arrangements.
I am sure if we had had children that we would have been happy and been excellent parents, but we are so happy as we are that I just don't feel any regret at all. To be honest I think we did IVF as I hated 'failing' at something that everyone else could do.
Our life means we can just basically prioritise having a lovely time on earth for however long we're lucky enough to be here for. Being happy in yourself is probably going to be the biggest factor in all this. Had I stayed with my ex-husband I would definitely be regretful, as I was so unhappy that a lifetime of just me and him would have been awful. I would have needed children as an escape!
As long as your own world is happy and fulfilling - whatever combination of friends, partner, work, hobbies etc. that is - I honestly don't think you can go far wrong.