I'm so sorry that you're going through this elizabeth (you are definitely
not worthless) - I can't speak for your mum and stepdad, how horrible of them to treat you that way - but on behalf of all of the decent and lovely people here on the forum, I think I can safely say that you are not a disgrace and you are definitely welcome here to express your feelings as much as you need to, as frequently as you feel.
I don't think that you are a 'demonic entity' but your posts speak to the fact that you have may demons in your life, perhaps in your past and definitely inside you right now, and by 'demons' I mean emotions and pain that only you could ever possibly truly understand. But that is why it's all the more important you feel able to express yourself, because the one thing that demons don't like is 'light', being out in the open where they wriggle and writhe in the gaze of people who care about your wellbeing (that's us).
None of us knows you personally and vice versa, but we read your posts and I'm sure many of us can relate a lot to the way you describe your feelings, the sense of desperation, despair and hopelessness come across strongly in your writing. I know that none of us can make your situation better right now, but please remember that we are here to listen, whether that takes the form of reading your posts, talking over a PM if you need that, or those of us who acknowledge you by sending a little hug or heart emoji.
No decent, rule-abiding member here will ever tell you to 'shut up' or call you selfish or in any way discourage you from expressing yourself, so if you genuinely can't express yourself to your mum, stepdad or any of your other family or friends in your life away from the keyboard, then please keep on posting here. If it's something that you don't feel comfortable about sharing with us, then just make a note on your phone (or computer) instead and pour it all out as if you were posting here, but with the knowledge that only you will ever read it.
I sometimes think that human emotions are like pressurised canisters, we all need to vent every now and again to release the pressure that builds up, otherwise, the metal casing (our body) begins to buckle and bend out of shape and hurt, and if we're not careful we can end up blowing a gasket, exploding (or more often, imploding).
So keep on posting, keep on venting and someone will always be around on here whether just to listen or to help you work through the issues in your life and try to re-frame them in a way that makes them seem less insurmountable.
Sending hugs