I was trying to do a portal run, then I died and later I checked the run's demos, and realized it cut off after I would have loaded the last autosave before death, how do I make it continue recording demo files after loading the save

it sounds like youre using the "record" command instead of the auto demo recorder. the auto demo recorder can be downloaded in the resources section, or it is built it and automatically set up in the Source Unpack version of the game which is highly recommended to install. if you already have source unpack do the commands: speedrun_save vault, and then use speedrun_start to start all of your runs going forward.