You are going to have to strip it down and replace whatever worn or broken parts have caused this situation. It's very ill-advised to continue using any kind of gun that discharges of its own accord, I am surprised you persisted. There may be issues with the piston rod, the sears, springs in the trigger mechanism or with lubrication. The dieselling, if strong enough, may have forced the piston back hard and the latch may have damaged the sear/bent.

You will need a spring compressor as the mainspring is under some tension on this rifle.

Parts diagram:

https://www.gunspares.co.uk/showcatimage.asp?id=24388