Ok, I'm not a hairdresser so can't professionally help you there. But coming from an outside perspective and meaning no awfulness here,you seem to be contradicting yourself and not seeing it yourself.
Ok so your wanting a hairdressers opinion but not willing to pay for this by going to a salon, right understandable you cant afford, you've been offered good advice to go to a college as they have the knowledge just new, but you dont want to go and rather do it yourself but your wanting advice on what to use?
Sorry hunny but you need to head the advice given previously by pros. YOU cannot save your hair with thinking like you are. If you persist to do your own colour ontop of previous theres a build up so you are making it worse with what you put on, see if you go to a salon/college they can say whats best, i personally never knew that putting a dark colour ontop of bleach never helped, i always presumed colour is best for your hair but not always the case. You could just get your roots done, highlights etc and cut off any dead ends and grow your hair but it has to be RIGHT. Dont think you can do everything yourself, you cant, you want your hair and doing right you pay for professional advice/knowledge NOT just the products.
I even know from years taking notice of my trusted hairdresser that any darker colour ontop of light will not stick the first time, It will take numerous times as it will fade quickly, thats what putting dark ontop of a light does, especially damaged hair ! Just think of it this way, would you really stop going to your doctors or dentist because you have google and can diagnose yourself ? No of course not, because they've studied years and know what there doing. X