How to test my thunderbolt port without device?
Just so you know the Thunderbolt ports on your computer have dual purpose, they are also Mini DisplayPort (ports). The cable you are using is actually a Mini DisplayPort to VGA adapter.

I've used these a fair bit at home and work, and at least the Apple ones aren't very durable in my experience, it's likely the cable has failed again. Unless you are finding other devices aren't working either, it's likely the cable.

You could book a Genius Bar appointment at an Apple Store and see if they can test the port for you with their own known good cables, that way you wouldn't have to buy another cable.

In future you could have a look at 3rd party products, StarTech adapters are pretty reliable in my experience but any brand would be worth trying.