Is the mechanical part of aerospace very hard to learn?

Hello Dominique,
I think that the most important factor in learning new material is having a positive attitude. If you learn with a positive mind you may discover that although a concept is hard when you first encounter it, you can look at many different examples of that concept which will help you understand how it fits into everything you already know. With mechanical engineering in particular, I think your real world life experience can support learning mechanical engineering. Have you ever built a birdhouse and seen how you can split the wood if you tighten the screws too much? Or have you built a sandcastle on the beach and seen that the walls are more stable when the sand is damp? Or have you built a paper airplane and seen how the flight changes when you bend the wings in different shapes?  Learning about materials and how they perform when you use them is an important part of mechanical engineering and you can learn a lot by observing the world around you before you ever get into a mechanical program. If you spend your time believing you can get over any obstacles and you observe the world around you closely, I think you will be able to learn everything you need to succeed.  Best of luck!
Jen