Hey. Happy nearly 40th birthday. I honestly think the only way to approach the milestones is with as much joy and celebration as you can muster.
I don't know what it's like to have a tough fertility experience - all I can say there is I'm sorry it's so hard for you, and I'm sure it must feel like the most important thing in the world to you and your husband. I've seen friends go through it, and it looks like hell from the outside. I wish you all the luck in the world, and I hope you get lucky and pregnant. For what it's worth, without exception, all of my friends were ultimately successful in getting pregnant and having at least one baby (and some took years to get there - one used donor eggs, another 4 x IVF), but I know it's not the same outcome for everyone.
What I think you need to ask yourself is how can I fill my life with meaning without having my own children? And there are so many answers to that - both involving other people's children or other routes to meaning altogether.
You say 'I don't know if we'll ever have a family' but you do have a family - your husband at a minimum, and I'm sure you have lots of extended family too. You can choose to involve yourself to the maximum with nieces/nephews/friend's children. I know it's not the same, but those relationships can be incredibly deep and profound. There are friends of my mother's that I turn to all the time as an adult for their wisdom and support, and my uncle was more of a father figure to me than my shit dad ever was.
I'm not being glib - of course I understand that it's not the same as having your own kids. But I believe there can be incredible meaning in the connections we make with kids, wherever we meet them.
Secondly, what else is important to you? What will make you feel proud of yourself and your contribution to the world? Can you volunteer, be the best sister/friend you can be, offer your skills to your community, make a creative contribution, add materially to someone else's happiness - work to understand what you can uniquely bring to the world.
We only get one go around. I'm sure like me, you have family and friends who haven't even made it this far. Make it count and know that you are incredibly important and valuable to the world - whether or not you are successful in bringing your own children to the world.
I wish you every success and a very happy birthday.