> It is very discouraging to work so hard on something and not do as well as you hope.

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This is grad school in a nutshell. Don't go if you don't want to because once you start you'll not want to quit and the costs of quitting will be greater. Turning down the offer now is less embarrassing than starting and then realizing that you want to quit but everyone knows you started, you paid the deposit and you're in the class and the leaving process is more formal.

You will have the UK equivalent of a 3.5-3.7 US GPA with a 2:1 Hons undergrad. So, of course, they took you, I got the same result and got into all the universities I applied to and just felt it would be stupid to not accept. But, it would have been easier to not go at that time than to go when I knew I didn't want to.