Internal insulation despite a damp wall

It's about an apartment and not a house. But I urgently need help!!!!
Hello, I moved into a new apartment with my children this year, which we had looked at last year while it was still occupied.
Before we moved in, the apartment was empty for 3 months over the winter, and when we arrived we had to find out that basically every room has mold.
We have a ground floor apartment, the basement is below us, and in every room there are exterior walls (we have the entire floor).
Our building must be at least 50 years old or older...
We have zero insulation. When it’s windy and I hold my long hair against the wall, it blows away.
I would like to insulate the apartment from the inside cheaply, since moving is currently not an option for cost reasons. I believe we only have the masonry, which has been wallpapered.
It’s almost freezing in the apartment every day, we have extremely high humidity in most rooms and therefore even more mold in the apartment.
For example, if I leave the window open for a long time in the evening in summer, creating a draft everywhere, the bed sheets are already damp again in the morning.
The current condition is very stressful because primarily I do not want to expose the children to any dangers.
If I don’t take the mattresses out to air them every other day, I have to fear that they will mold.
I think the strong humidity comes from the constant warm-cold cycle, right?
Today, it was about 15-16 degrees outside; therefore, the apartment was at most 18 degrees. If it’s 30 degrees outside during the day and the whole apartment is warmed up, it immediately gets cold here as soon as the temperature outside drops. (So at night then)
Please, can you tell me how I can insulate from the inside without causing even more mold in the apartment later (for example behind the insulation)? It should be as inexpensive as possible.

I have to dress my little daughter in 2 pajamas every night in the summer and put her in a thick sleeping bag so she doesn't get cold :(

I am just wondering whether moving wouldn’t actually be more cost-effective. Insulation + new wallpaper is definitely not more expensive than moving. Especially since it’s not even clear where the moisture is coming from and the actual cause has therefore not been resolved. I would first talk to the landlord and demand a remedy and reduce the rent until then. If he is stubborn, immediately start looking for a new apartment.

Health is priceless! Staying in the apartment long-term will probably make you sick.

Draft no longer than 10-15 minutes. If you ventilate longer, the walls will only cool down more and moisture will condense better/faster.

The landlord does absolutely nothing. We bought a very expensive kitchen here, custom-made specifically for the apartment :( I also already had a conversation with a lawyer... She said: the landlord is not obliged to insulate the apartment. I just keep a longer draft during extremely high temperatures so that nothing cools down. What about insulating plaster? Can no one give me some advice?

First, the cause must be found. Fluttering hair can also be caused by air turbulence (cold wall, warm air) or leaking windows. The moisture can also come from an interior wall or the floor (leaking pipes). First investigate the causes and then take action. In case of mold infestation and moisture, the landlord must take action. Please contact a tenant protection association.