When ever i try to save a mod that i previously started and have saved - autosave won't work and I'm unable to save the it to the same mod. I can save it to a new mod name but when I do it only saves the new things and everything else that I've done previously isn't contained in the new mod. Does anyone else had this problem? Please help

You mentioned Autosave, are you talking about saving mods you are playing in the game or mods you are making in TESCS?

in the TESCS:
When you load your data select "Oblivion.esm" (original file) & your mod file .... for example "My Home Mod.esp" now select your mod file and push the "Set as Active File" button. this way you make it active and save all changes to your mod file 8)

Thanks