DDOS mitigation requires a lot of networking and server knowledge and it's not just a "software to run on a VPS" solution. If it was that simple, we would not have DDOS attacks around the world in the 1st place.
As such, there is no software to protect you from DDOS, and that is why it's only provided as a service from very large providers like Cloudflare or AWS, etc. that have the infrastructure to combat such situations. Your best bet is to talk to your VPS provider and work on a solution or to a network engineer, that you can find on dedicated networking/server sites and see the advice they will offer you.
Since there is nothing else to add to this, the thread will close to avoid pointless chit/chat and I hope you find a solution to your problem.