Alot of those symptoms of your list there sound like what I do on a daily basis..
- I talk under my breath at times - trying to remind myself to do something, or just sometimes without thinking about it.
- Its not uncommon to pretend within yourself that you're someone else - someone better, but maybe you should take a look at why you wish to be someone else.. is it someone more popular for example?
- Feeling socially isolated is worse for some people than others, and I know how horrible it can feel. But the longer you stay out of social situations the more intimidating they can seem and the more the fear becomes what keeps you from going back into the situation.
- As for the fear of spiders - I think a good majority of the British population has some level of fear over spiders, and it’s pretty much all irrational.
Some of the other symptoms.. such as holding doors open for people who aren’t there and hearing voices may be a little worrying.. but as has been mentioned it could just be your mind manifesting symptoms to help support this diagnosis you think you have found.
If you are really worried - go and see your GP and maybe take someone with you for support - family member or close friend, I know when I go to the doctor I tend to only hear what I choose to hear so its good to have someone else there to remind you of what’s actually being said.
Good luck to you. I'm sure you'll be fine. And if it does turn out to be a level of schizophrenia, it is quite treatable now-a-days with medication and such I believe.