What is the ruling on swallowing water that collects in the throat during fasting, which resembles saliva but may have come up from the stomach and sometimes has a taste? Does it invalidate the fast if swallowed, given that it does not reach the mouth? Is it considered qals (regurgitation) that invalidates the fast if it has a taste? Is it permissible to ignore this water and swallow it due to hardship, and what is the ruling on intentionally burping to see if anything comes out with it?

It appears that what you are suffering from is obsessive-compulsive disorder (waswas), and we advise you to ignore it and turn away from it. It seems that what you are spitting is ordinary saliva. Even if we assume the truth of what you mentioned, as long as you feel it in your throat and it has not emerged to the outer part of your mouth, there is no harm in swallowing it. There is also no harm for a fasting person in intentionally burping. However, if anything comes out to the outer part of their mouth, they must spit it out. There is no better cure for waswas than turning away from it.