Hi mega_biscoito.
Welcome to the Mattress Forum all the way from Europe! 
Thank you for your stats and the brief mattress experience you shared with us.
You did not specify which mattresses you tried but based on your description it looks like you have purchased memory foam/ pocket coil mattresses. It is unlikely that the “softening” of the beds is because of the pocket coil units. Most likely culprits for softening in the first 2 weeks of purchase can be a result of the “breaking in” of the new mattress and/or having low-quality memory foam or polyfoam in the comfort layers of your mattress. When you lie on a memory foam mattress, it will soften with heat and pressure and become semi-solid. It is also less open-celled than other foams so the air inside the foam takes more time to both move away from pressure and also to come back “on the rebound”.
It is difficult to control how far you sink into a memory foam layer as it may soften over the course of the night and what started out as correct spinal alignment could turn into a “hammock” position and a backache by morning as you noticed after sleeping on your stomach.
For your stomach sleeping position, you would need a flatter, firmer surface than most sleepers. You may wish to review the guidelines in this Sleep Positions Article so that you avoid hyperextension in a swayback position that can cause back issues.
[QUOTE] Can you advice me any mattress, according to what I wrote? Which kind of mattress do you recommend for me? Which aspects should I consider to buy the mattress I’m looking for?
[/QUOTE]
The first place to start your research is the mattress shopping tutorial here(there is a condensed version of it at the end of that post) which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps, more importantly, know how and why to avoid the worst ones. Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well he will sleep), durability (how long he will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
The weak link of a mattress is almost always in the use of low-quality materials in the comfort layers and if you don’t know the specifics of what is inside your mattress you have no way to identify any weak links in the mattress or make meaningful comparisons with other mattresses so you would be making a completely “blind” purchase. When assessing any product, also be sure that you find out information listed here so you can compare the quality of the materials and components to the mattress durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
Before you venture on purchasing a new mattress, I’d do a hard reset and arm yourself with the information you need to know I linked above. I’d first determine the type of mattress and the feel of the material you like, (this may involve testing some mattresses locally). It may still be that a good supportive pocket coil with the right combination of comfort/transition layers can work well for you and your partner. You did not mention your partner’s primary sleeping position which can also change some requirements. Then I’d focus on the quality of materials within your mattress to determine how long you can reasonably expect to sleep well on the mattress.
I’m always happy to help the forum members here with “how” to choose, to act as a "fact check, or to help them find better manufacturers or retailers that have better quality and value but “what” to choose in terms of either a mattress or the balance between comfort/pressure relief and support/alignment that may be suitable for a specific person is always up to the only person who can feel what you feel on a mattress and best left to personal testing and your own experiences or more detailed discussions with the more knowledgeable people at the retailers or manufacturers that sell a mattress you are considering.
I hope this gives you a good idea of how to start looking for better options and what to look for in your search for a new mattress.
Phoenix