Termination during notice period and experience letter

Hi guys, I have a question, I'm serving 3 months notice period of which 3 weeks I have served. I resigned because the project is complicated and deadlines are strict which was causing stress and bad mental health for me.

My question is that if I do my best, but because of complexity, not able to deliver work , can the company fire me and if they do, can they hold my experience letter, I don't care about money, I just want the experience letter, I'm even ready to buyout notice but I don't think they will agree, they want to suck me dry till last day even though I have told, the project is too complicated and I'm not getting it. If I maintain good behaviour and show up, but even after doing my best if I can't deliver, can they do something with experience letter?