I'm in the process of trying to come up with an idea for my phd but every time I think I have something, turns out someone else has already done it! I'm just trying to keep reading as much as I can but it is a bit disheartening when you think you've finally found something only to find out it's already been done, starting to lose hope that there is any original ideas left out there or that I'm intelligent enough to think of one!
Any suggestions of how else to come up with this elusive idea would be very much appreciated!

You don't have to change the world- just make an original contribution to knowledge. You can be very niche.

For example, if you are interested in Charles Dickens, you could do a PhD about his moustache length.

You need to consider what your undergraduate and post graduate qualifications are (ie. which sub-discipline of social sciences you studied). You also need to think of what are you passionate about, for example, is it refugee rights that you care about? That is what you should use to guide your choice of topic and think about how you will make an original contribution.

Sometimes originality comes from the approach you are taking to making links between different things - for my PhD, the novel aspect is in the combination of factors that we are considering in the context of two problems. They have individually been looked at extensively, but little has combined them.

Maybe you need to have a closer look at a specific problem, and at some of the proposed answers/solutions to that problem? If it has all been said already and you need to say something new, build those established answers into your thesis and argue against them - what are the alternatives? Is there another way to approach this problem? What is a different angle? Have the methods others have used been quite similar, is there any way you could approach it using a different methodology?