Easily broken screen

I took my phone in so the screen could be replaced on the 25/10/2018 however my screen broke on the 30/12/2018. The device was in my pocket and when I took it out the screen had cracked. I do not understand how this occured if the parts being used to replace my original screen was a good quality. I have called the head office and I was told that I would get a 5% discount on anothet replacement and the fault was mine as I was negligent. The store advises a lifetime warranty but now when they have to replace the screen due to the part being a not so good quality the blame is being shifted to me. I am demanding them to fix my screen on their account and further more expain the type of screen that was used and original apple screens are of good quality.