Is building a house feasible with my budget? Or should I give up the dream?

With a 350,000€ loan and 1% interest, you’ll be done in 28 years.

I would aim for a bank appointment: in my opinion, you won't get a loan alone, 1% interest is also unlikely, but you have to create facts sometimes.

reexamine the topic of partnership.

What is her stance on the topic of "moving into your house"?

It's nice that this is brought up. Personally, I think partners are very reluctant to get involved in something that runs parallel without them. One emotionally turns away from the project.

I am always surprised what unfeasible prices are quoted to the original posters here, or that basically nothing is possible under such and such a price.

For God's sake, I don't want to attack anyone here or anything. I like to read along here. But it does surprise me.

Let's take a classic example, the Town & Country Flair 110, which also matches the square meters the OP has in mind. He wants to build in RLP, I haven't read about a specific area yet. If I look at the prices for the Flair 110 from Town & Country and take the cities Mainz, Koblenz, Mayen, prices from just under €180,000 to around €201,000 are shown.

A Flair like that is built by the hundreds and it seems to work. A neighbor here also built with Town & Country, for example.
When I see that for my 100 sqm house (not Town & Country, but Heinz von Heiden) about €163K was charged including customizations. Ok, that was 2018 and yes, it has gotten more expensive in the meantime, but not so much that you have to add €100K or more. Each electric roller shutter was about €300 on the extra price list, additional sockets by the general contractor electrician about €40, flat shower tray €500. So all within a certain range. The staircase, for example, as final treated including metal rods and straight newel post about €650 (settled directly with the general contractor's stair builder, just final treated would have cost about €800 through the general contractor).
If you don't have luxury demands for yourself, I think it should be possible within a manageable budget. You don't have to badmouth or talk down the "big affordable ones."

Just my 2 cents

If I see that I get for my 100 sqm house (not Town & Country, but Heinz von Heiden)

Heinz von Heiden should offer the same standard at a similar price as Town & Country.

I don't know how it is now, but back then Town & Country, for example, only had one solar system as standard, Heinz von Heiden a heat pump. It would have cost several thousand euros extra at Town & Country.

I don't know how it is now, but back then Town & Country only had one solar system standard, Heinz von Heiden a heat pump. It would have cost several thousand € extra at Town & Country.

The bottom line is the price!

I am always surprised at the impossible prices that are quoted here to the TEs, or that basically nothing is possible below this and that price.

For God's sake, I don't want to attack anyone here or anything. I like reading along here. But it does surprise me.

Let me take a classic, the Town & Country Flair 110, which also corresponds to the square meters the TE has in mind. He wants to build in RLP, a region I haven't read yet. If I look at the prices at Town & Country for the Flair 110 and take the cities Mainz, Koblenz, Mayen, prices of just under €180,000 to about €201,000 are shown.

Such a Flair is built by the hundreds and it seems to work. Also, a neighbor here, for example, built with Town & Country. When I see that for my 100 sqm house (not Town & Country, but Heinz von Heiden) about €163K was called including the upgrade. Ok, it was 2018 and yes, it has become more expensive in the meantime, but not so much that you have to add €100K or more. Each electric roller shutter was about €300 on the price list, additional sockets by the general contractor electrician about €40, flat shower tray €500. So all within a range. The stairs, for example, as finished including metal rods and straight exit post about €650 (settled directly with the general contractor stair builder, just finished would have cost about €800 via the GC). If you don't have luxury claims for yourself, I think it should also work within a manageable range. You don't have to badmouth or make excuses for the "big price winners."

Just my 2 cents

You certainly don't have to make excuses, but you have to stay realistic. 100 sqm is nice and great, but if a family is supposed to move in, you have to be aware that it will be cramped. It does make sense to coordinate such a project with your partner.