I am so sorry that you are going through this, it sounds absolutely horrible and like it is so much to get through each day. I have a suggestion that may or may not work for you, that is, what if you agree with your parents, and that these issues are induced by your "bad mentality".. would they take you to see a GP so that you could have your "bad mentality" addressed and have someone to help you "fix" it? This may be the way to get the help that you need from a GP and to have some tests done that eliminates any stomach or digestive issues and that they can suggest it might be to do with anxiety and stress and can help you with that. This way you get to see your GP, you get the help you need to see if there is an issue internally and also some help with coping with these anxious moments in your life that you live with each day.
You are so very right in that you are not able to remove your father from your life easily, well not just yet but what you can do is perhaps change the way that you interact with him and communicate with him for a better outcome. You could even write your father a note if you feel too scared to talk to him and let him know that you do feel this way, that you are scared and that you do feel uncomfortable, that you are trying to manage your feeling ill but the pressure of managing it with no support makes it worse. That you are not intentionally going to the toilet all the time to upset and anger people and you are being made to feel like it is something that with a click of the fingers it will all go away, that being the case you would have done that by now. What you have written here explains very well how you are feeling and what you deal with each day, is it possible that they actually don't know the extent of your pains and your symptoms? Maybe you could even write to them something similar to what you have done here.
I think you are in very much need to chat to your GP and I hope you can get an appointment so you can get on track to starting to remove these things from your life.
I am so pleased you have reached out for some support and I hope to chat to you some more, if you feel like it, I would really like to know what you think of my comments.
Hugs to you Kairos