Dissemble sixaxais controller fully

I have an old sixaxis ps3 controller that has sticky rubber thumbsticks. Apparently these controllers have a clamp that is right below the thumbsticks. This controller has never been taken apart before.

I have removed the 5 screws in the back, but I can't get the clamp undone at all. Is there a special tool or technique that I could use?

The clamp is really hard to undue.

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I finally managed to get the controller apart but I having problems with reassembly. I'll see what I can do.

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After spending hours with this, I was able to take apart and put back together the controller. Very difficult to do.

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