[2010] XNA - 2D Platformer Engine
Back then in 2010 I found out there was a Game Development framework called XNA (by Microsoft) that allow people to create games that run on Windows and XBOX 360. The programming language for this framework was C#, so I was pretty happy since I used that in my job by that time.
I created a big well-structured platform engine with so many classes and helpers (I also built a TileMapEditor in XNA to create the world maps and collider maps).
I spent more than 2 years developing the engine and suddenly... Microsoft was no longer supporting XNA. XNA was dead, it was a huge disappointment since everything I develop was useless.
Later I realized even that my project was truly dead, I learned a lot and started to look for some other technology to continue my GameDev journey...