I have to admit that for most of my career I didn't think my books had any structure at all. But I recently noticed that I've been using three act structure for quite a while.
First, what is it?
Your book begins with an inciting incident that gets the characters together. But by the end of act one, usually chapter 3 for me, everything changes.
In NANNY FOR THE MILLIONAIRE'S TWINS the heroine tells the hero she is engaged.(That'll change a romance.)
In THE TYCOON'S SECRET DAUGHTER, the hero has demonstrated that he's a totally different man than the man the heroine was married to. No longer drinking, he's like a brand new person and she has to deal with him that way.
Act II has the characters dealing with this change. A lot of times that change causes work and/or new goals. For instance, in THE BABY PROJECT, the hero and heroine who have been given co-custody of a baby move in together. Act II is all about the changes that happen when they move in together and try to raise this baby together. He has to learn how to change diapers. She has to deal with a baby when she's still hurting over the loss of her own child.
Of course, in that time together (Act II) in a romance, the things that they do to solve the problem discovered at the end of Act I, will bring them close enough that they will fall in love.
But...at the end of Act II there is a black moment. A point in time when these two people who have been falling in love suddenly realize this isn't going to work.
Now, all black moments are different. So sometimes your heroine will think it's working but the hero freaks out ... or sometimes the hero has a secret that when discovered ruins everything. There's no pat answer for a black moment, except to say that it DEMONSTRATES (not tells but shows...demonstrates) to the H and H that maybe this love they think they have is wrong.
Which means there's misery in Act III, along with some soul searching until either the hero or heroine realizes they were wrong in thinking this love wouldn't work and one of them goes back to the other and professes love or explains why this love will last.
That takes us to the happy ending.
Now, I'm sure others may draw up three act structure differently. But I've hit the highlights of what I do. (And it's worked for 54 books! LOL)
Happy Monday! And thanks to Anne for the question!