How to afford building a house and buying land today?

We actually had ours for 20 years

...and then someone took it away from you? That must have been terrible

Am I willing to live in rent at the bank for decades (for most 25 to 35 years) and therefore give up freedom, vacation, and the good feeling of owing nothing to anyone?

This question is very good and has nothing to do with fear.
The term "freedom" is especially close to me. Freedom begins in one’s own mind. Regarding "living," owning a house does not make me more or less free than renting one. The money goes one way or the other, and I can change the condition and location if I want.
The "good feeling of giving something up"—I am still practicing that and smile at my self-deception in my attempts. The engagement with renunciation has many good aspects.
The "good feeling of owing nothing to anyone" is an interesting category. I approach this more holistically: I think that as a social being, a person needs a form of interdependence so that society and peace can function long-term. The goal of functioning as an unbound, self-sufficient unit may be individually desirable at first, but socially it is questionable. People always need help. This includes the qualities of being able to give as well as to receive. Whoever gives without being able to receive creates an imbalance. Entering into obligations with each other is social glue.

Unfortunately, in reality, you can’t just sell your house like that. It’s like with a new car: driven one kilometer and it’s only worth half because it’s used. Somewhat exaggerated, but with a house it is similar. The ancillary purchase costs are definitely gone. And the bank will, if necessary, also want its prepayment penalty.

That may be so, but it doesn’t have to be. We are currently selling our townhouse with a good 100% markup over the total construction costs from 2001. Could have gone worse. You might lose some money. So what? That’s just how it is when you realign your life. Imagine how many people it goes like this when they separate? All doable.
Only not having money and not being able to meet obligations is a tyrant. Everything else you can decide either way. Money comes and goes. Quality of life and time do not. This thought greatly simplifies setting priorities.

In the end, building a house is just building a house and not something a person must do or refrain from doing. For some, it is the fulfillment of a great inner need; for others, a pragmatic cost/benefit consideration. Others avoid it because they don’t want to tie themselves to property and loans.

Am I willing to live for decades (for most people 25 to 35 years) in rent at the bank and therefore give up freedom, vacation, and the good feeling of owing no one anything?

Where do you get these fears from? Compared to living in a rented apartment, I feel much freer now in my "own" (in 26 years^^) house. Vacation is not given up either. We probably won’t fly abroad once every year and take 2-3 short trips like before, but 2 vacations a year are still doable, and starting next year, when the outdoor facilities are finished, long-distance trips again. Everyone just has to know for themselves what is important to them and when.

Compared to living in a rental apartment, I now feel much freer in my "own" (in 26 years^^) house.

that's exactly what I hope for with my own property as well; compared to a rental apartment, I am simply much freer in what I can do with it. I can influence the layout, within certain limits also how it should look from the outside. everything exactly according to my ideas. I don't have to live with whatever toilet my landlord installed. in x years I can also have the kitchen connections moved if I want a different shape. I can add windows if it's too dark for me. and last but not least, I have a garden right in front of my house (well, not everyone wants a garden , that also means work). if after 5 years I don't like this or that anymore, I change it! I don't have to move completely for that. that means freedom to me. and how tightly everyone ties their so-called "corset" for themselves and how much money they then plan for vacations is up to each person. if the desired vacation is a must, then you might have to make compromises with the house, that's called "clarifying your priorities."

The "Freiheit Mietwohnung" will probably refer to the fact that one can break camp and move at any time. If that is important to you, you should not build a house. If you want to invest, an apartment is suitable, as you can rent it out at any time – I wouldn’t do that with a house.

Americans set an example. You can break camp at any time, even with a house.