Hi Antiopi Kyriakakou, you can find a list of discussions by selecting Report along the top bar of the space. Go to Content along the lefthand side. Once you have done that, you can change the date range (up to a year). You can download a few different reports to capture multi-year content.
If you scroll down a little bit to the list of top content labeled Content Leaderboard, you can select Download CSV on the right. That will give you a complete list of all content types. In Excel, you can filter content type by discussion.
Here is list of top content where you can download the content CSV: