Hi koko1,
It’s possible that the temperature in the store was higher than the temperature in your bedroom which could also affect the firmness of the memory foam in your mattress.
The mattress in the store was probably already broken in as well so it’s also possible that your mattress will soften over the first few weeks and feel more like the mattress in the store over time.
There are also some suggestions in post #2 here that may be helpful.
It’s also not unusual that some people will sleep warmer with memory foam (or gel memory foam) … especially with mattress protectors that have “semi breathable” membranes that can reduce ventilation and airflow so while they would generally be fine for the majority of people … there are some people where this type of protector in combination with some of the other factors that can affect sleeping temperature would be enough to put them “over the edge”. There is more about tracking down sleeping temperature issues in post #2 here.
There is also more about the pros and cons of different types of mattress protectors in post #89 here but if waterproof or water resistant isn’t an issue for you then a stretch knit cotton protector would be an option that would have very little effect on your mattress.
Phoenix