What is the ruling on forging a certificate and practicing a profession with it to please one's mother and avoid hurting her?

Filial piety is great, but there is no obedience to them in disobeying the Creator. You erred in forging the certificate, and it is not permissible to work with it. If you find a job with your experience without submitting the forged certificate, there is no blame on you. As for your mother's request, you can gently make excuses to her without lying or displeasing her.