Your vocal folds are like most moving bits of your body - they need to be warmed up before use. Straining them can give short term problems like a sore throat and croaky voice, or they can be injured. Then you start to develop little lumpy bits on them called nodules. These often need surgery to remove, and very rarely is your voice back to where it was - it changes, hopefully, subtly.
Extending your range can be just something that happens in the throat area, or it can be something that happens lower down, in the diaphragm region. Thing like an organ pipe - the little wedge shape slot is your vocal fold, but the overall tone and range is produced by the rest of the pipe - it's shape and rigidity.
Singers with long careers have learned how to support their voice and look after it, running it in properly like we used to do with cars. Many popular singers have short singing careers because they abused their voices, accidentally, and after a while, like wrinkles, the vocal folds just can't hack it any more.
The answer to your question is a simple one - yes, you can extend your range. How much by, and for how long needs a professional input who can listen to you and advise. One singer I know well, who was in Les Mis in London, moved to Broadway, and is now doing the same role in the Philippines. He sings powerfully and every single day, twice! He has a practice regime recorded, and it lasts about 15 minutes, and he does the same exercises before each performance. He has a bong like gizmo he inhales vapour through to saturate his vocal folds and prepare the skin inside for the workout it's about to get. He rarely has to take a show off - his voice pays the mortgage. Another friend has been singing for nearly twenty years and her voice is getting tired, despite having singing lessons - mainly because she often has to abuse it. She works on cruise ships, and extra shows when she is tired get sprung on them, or perhaps she has to sing louder than usual because an alternate location means everyone turns up. Unlike Broadway/West End, protection of performers voices is not high on the list. She's had nodules removed and her voice is wearing out early. Still good, but not as good as it was, and she's not 40 yet!
You have already said you've picked up bad habits. Getting rid of them suggests the thing you really need, is what you don't want to get - a professional. If you were a sportsman, and wanted to improve your times, would you experiment with your muscles and joints yourself? Maybe stretching ligaments or injuring joints?
