You are going to hate (and I mean HATE) my advice but I’m going to tell you now- I’ve seen this work and it will be one of the hardest things you’ve ever done.
Write him a letter, tell him you were wrong and that he’s an adult and that you were so sad that he left but that you understand this is his choice and that you support him and his relationship. That you realize now that you were reacting from a place of fear and confusion, but you see how committed he is to this relationship and you see how committed his girlfriend is.
Now before you freak out at me – is any of this baloney true – fuck no. Not at all. But here’s the thing… your kid is 19. He’s bought into the Romeo and Juliet nonsense where you and your family are the evil Montagues but her family is some weird version of the Capulets where they’re just fine with everything. So your first step – and this is critical – is to take the barriers away. Don’t ask him to come home. Don’t ask him to stop seeing her.
When he tells you how happy he is, tell him how happy you are for him. When he tells you that they are engaged tell him that’s wonderful and you’re happy that your family is growing. THESE ARE ALL LIES. These lies keep the lines of communication open with your kid. When he complains about her – and he will – you say NOTHING bad about her. You say things like “relationships can be hard, but communication and compromise are key” or “each person has to give 100%” – things that sound supportive but give him a little reminder about what he’s not getting. Partnership, common goals, shared responsibilities, whatever you can insert to help him understand that he’s made a terrible mistake but without telling him he’s made a terrible mistake.
And I’m not lying when I say this is going to be the hardest thing you’ve ever done -but you have to treat your son as if he’s a bitchy coworker who always picks the wrong partner and then complains that the partner is wrong. You have to put seventeen dampers on your emotions because you will want to yell at him and beg him and everything else but if you do that – you’ve shown your hand and it’s all over. Seriously consider getting some anti-anxiety meds to help you through this.
It’s time to treat your kid like an adult. All you can do now is support him and subtly let him know that you’re still there for him, but in a different way. Good luck. This sucks.