I have made a big mistake by signing up for a contract yesterday. I was not told that I'll have to handle so much work and now I cannot keep up with my client. I want to end the contract but I fear my 100% Job success rate profile will be shattered in pieces because I failed to deliver and my client will submit a bad review.
Please guide me how to proceed.
I know it's so silly of me to ask but please help me out.
Although the Job Success Score has some silly elements, it isn't at all silly to worry about your Job Success Score when Upwork automatically defaults all clients' new project to select for freelancers with a JSS of at least 90.
It sounds like you need to communicate with your client and tell them you misunderstood the project and cannot completely to your high standards. If you do this immediately, a good client will appreciate your honesty and your quick notice, allowing them to quickly decide if they want to accept changes that make the project doable for you or select another freelancer to complete the project.
Any delay in notifying your client will not work in your favor. Let your client know immediately and tell them what changes you'll require, if that is a possibility. Make sure your phrase it as a request for changes, not a demand. Let them know you accept the mistake is yours (even if it wasn't completely) and you appreciated your acceptance of you previous proposal.