I can see both sides to the comments on being enmeshed.
From reading your post I did instantly think: wow, you're overly involved in your daughters life and are trying to shelter her from experiencing normal human adult emotions, which is very unhealthy.
On the other hand, I also have a younger daughter who at 5 years old already has some very big emotions and needs a lot more guidance on how to handle those emotions than other children her age. I already know that making sure she grows up to be a happy, confident young women is going to be a challenge. I understand how you end up where you are right now.
But it's still not healthy. You're trying to protect her from having any painful experience. Not only is that completely unreasonable, it may also give your daughter the sunconscious impression that you don't trust her ability to overcome these things.
All heartbreak hurts. But people survive, they learn, they grow. None of this is dramatic, none of this is the end of the world. As parents, what our children need most is for us to be the voice of reason. They need their parents to say: yes, this hurts. But anything good comes at a risk of loss. That's how life works. Keep opening yourself up for beautiful things, and take the losses with strenght and dignity. I trust in your ability to overcome and heal, and more good things will be waiting for you.
They absolutely don't need you to panic because they may feel down for a while. She's allowed to feel down when she gets her heart broken. And she needs you to trust that she'll get through it, as any normal 19-year old would.
And in this way, you are too enmeshed in her life. Because rather than taking a step back and watching her have these emotions, you're having these emotions with her.
You've gotten fond of him.
You find it hard to watch her wait for him.
You like him
You don't want to see her heart broken
You hope they can work it out.
The problem is your fear of your daughters emotions, and this fear is affecting you way too much. And it's also hindering your ability to be there for her in a realistic and constructive way.
She doesn't need you to hope it works out for them. It's normal for her to want that. She's the one in love. You should be the one to tell her that you believe she'll get through it, no matter what happens.
So take about a dozen steps back, and realise that your daughter having emotions is not a problem, and it doesn't need to be made bigger than it has to be. Stay in your own neutral space. Be her cheerleader. Believe in her. Tell her it will be alright, and believe it.