It can be difficult to let go as we have put so much work into it and we want to succeed! Saying that, at this stage, it is unlikely that any changes you make will have a huge impact on the thesis and it sounds like it's bringing you down and even making you doubt your own abilities. Let's be honest, your supervisors wouldn't let you submit unless they thought it was good enough! So it may be more helpful if, like Ady suggested, you make a list of things you want to do before submitting and self-impose a date. Sticking to doing only the work that's on the list will help you to focus on the task as well.