Congratulations OP.
There will always be people who can't go to your wedding, especially if you need people to travel abroad for it. My husband and I am from different countries so wherever we chose to get married half our guests were going to have to travel from the other country. I had one friend with small children decline the invitation because it was going to be too expensive and complicated for them as a family, another friend whose wife dropped out a week before because she was in early pregnancy and feeling rough, and a cousin who just didn't show because his anxiety got the better of him at the last minute.
I also declined an invitation to a wedding that was a few weeks after my due date and would have involved travelling. I sent them a nice present instead.
On the flip side, I also put off TTC for a few months to attend a wedding in another European country, but that was my best friend's wedding and I was a bridesmaid.
At the end of the day, it's lovely if everyone pulls out all the stops to attend our weddings but sometimes it just isn't possible. And if you avoid TTC for two or three cycles every time there's a wedding you might want to go to, you'll be not trying more often than you are trying.
You haven't done anything wrong. Just wait until you're ready to announce your pregnancy to people, and then maybe give your friend a heads up before you post on social media if you are planning to do that. You don't need to tell her before you're ready if you're someone who wants to wait until after the 12 week scan to announce. Her wedding is next May, she won't be finalising numbers yet. Don't apologise, don't justify your decisions, just say, "We're expecting a baby around the time of your due date, we're really excited but unfortunately it does mean that we won't be able to come to your wedding. Hopefully we can celebrate with you at another time though!"
She might be disappointed but it's not reasonable to expect you to time your family planning around her wedding, and the only acceptable response from her really is, "Congratulations! Obviously we'll miss you at the wedding but I totally understand!"
If she's having a destination wedding or even if one of them is Italian and it's the most convenient location for most of the guests, there will be some people who will decline simply on the basis that it's in Italy and they don't want or can't afford to spend their time and money travelling there. And that's fine. It's an invitation, not a summons.