So I think there is a lot of exaggeration here.
Personally, at 19, just hearing the word "forever" in connection with a relationship would have caused me massive shortness of breath and tendencies to flee. I didn’t even have my high school diploma at that point... And otherwise, I have always been and still am someone who takes rather too much time than too little with long-term decisions and thinks everything through very carefully. For me, at 19, such a decision with such far-reaching consequences would never ever have been an option. Alone because at that time I didn’t even remotely know where my (life) journey was going. And many people might feel the same; maybe not as extreme as I experienced it (I personally didn’t want or couldn’t settle down privately or housing-wise until the beginning of my 40s), but the idea of having practically planned your entire life until the end at 19 might scare many. Me too. For me.
But I also definitely know cases where a couple found each other very early and already set the course for their future together very early. And not all of those failed. I would say: just as many as couples who made and lived those decisions later. It can work, but also not. The divorce rate in Germany averages around 53%.
Advantages of the "early" couples: by their early 40s, most of them are done with everything; kids are past the worst stages, the house is mostly or fully paid off, and they are professionally well established. That has its charm too!
As I said, the idea of committing so firmly in all areas already at 19 is a nightmare scenario for me personally, really scary. But if it’s okay for you, then go for it!
You still have a lot of time ahead of you; I would definitely choose the longest possible financing and keep the monthly installments as low as possible. Have the option of changing the repayment calculated in; that can also compensate for times of raising children.
Whether it’s the house now, I can’t judge that, but if you’re sure, then get married, buy the house and build and pay it off comfortably over a longer period. That is the advantage of your youth: you have time.
Divorces also happen to couples who are older. I would say: you grow and develop together when you’re young. Older couples don’t have that chance.
It certainly makes sense to already consider now what happens if we separate. (Prenuptial agreement, will). But that’s advice for everyone in the situation, not age-dependent.
Your advantage, BECAUSE you are young: it doesn’t have to be this house. You still have time to look. Reflect again whether you really want permanence, or if you want to save up some equity and maybe buy/build a house in a few years with better starting conditions.
I wouldn’t rush; but if it’s your dream house, then take it.