The requirement changes are driven by marketing and engineering teams. As a tester, you can definetely tell how the testing schedule and the efforts are going to change if the requirements are changed very frequently.
Looking at it from a QA perspective......they are changing frequently because you didn't do a good job inspecting the requirements. Examine every change and then try and understand how your inspection missed each critical defect. Make necessary changes and make sure those oversights don't happen in subsequent projects. Do the same for other teams...i.e. PM, R&D, UI, etc.
I could go on w/ other ideas but I won't....propose an idea and ask for suggestions....don't just take the easy way out by asking questions cause you're too lazy to even come up with 1 idea on how to fix this problem.
Reserve a few months every so often and preview retirement throughout your career. You won't regret that a 35 year career was reduced to 34 years to take vacations measured in months in order to remember what a stress and care-free life is all about.
Books and hard work will get you anywhere you want to go.
A lot will depend on how change is controlled within your project. If change is controlled and well documented you have a chance. Otherwise it will be very difficult to ensure thurough testing.
If change is logged and managed properly you should be able to assess the impact on your use cases and determine which need to be changed and/or rerun. Whether you are given sufficient time and resources to do this is another matter.
If change to requirements is wild and uncontrolled then, as kenyo suggested, talk to your manager. If you become unsure of what you are actually testing confidence in the results will be poor and you will waste much effort dealing with non issues. Make sure the problem is known and appreciated by others.
Change is inevitable so you will need some mechanism to deal with it.
The story so far:
In the beginning the Universe was created.
This has made a lot of people very angry and been widely regarded as a bad move.