Sounds like a very difficult situation all round. Sorry to hear it's been that difficult for you I really am.
On the plus side you're reaching out to this messageboard and presumably other places and getting curious about what exactly it is you're dealing with; whether it's one thing or multiple issues that cross trigger each other.
I'll re-read what Robert wrote as you said you related a lot to it. (I must admit I haven't even tried the parocpresis hypno link he recommended but I will.)
The bit where you said you can't poop if other people are in the same house sounds like parcopresis to me: The need to be completely alone before your body will relax enough to get the urge to poop. The feeling/reality of incomplete evactuation might be related to the parcopresis but see below on that one.
While the other symptoms you mentioned: incomplete evacuation, feeling like you haven't gone for days, feeling of it being stuck/blockage, smelly gas, bloating from trapped gas and the sharp pain that goes with it sound more like a gut problem like either IBS (irritable bowel syndrome) or possibly a gut disease like crohns, coeliac, or colitis. BUT I'm not a medical doctor. All I would say is if it's an option for you, and as much as you can, to start engaging with a medical doctor and getting tests and investigations done to rule out certain digestive diseases and/or diagnose one, it would really help and be a valuable investment in yourself. I wish I had back when I was your age. The test results and investigations won't happen overnight, it's more of a marathon than a sprint, but you'll be able to slowly build up an accurate picture and knowledge that will help you move forward.
I'll leave it there for now.
You're going thru a rough patch and I realise it's been going on for a few years now, but it gets better. The fact you reached out and posted on this dead old messageboard is more significant than you might think because it suggests you're able to be curious and interested in what you're dealing with even in and amongst the pain and distress that you're dealing with on a daily and chronic basis, and believe me that is a crucial assest.