Oh, sweetie! I feel terrible for you! My daughter started having constipation at 1 year old. She does not have IBS. I took her to a GI pediatric specialist, and she was put on polyethylene glycol. It "cleaned her out", and then she took it regularly for several months, and she would "go" about twice a day. What happens when you have been chronically constipated for that long is that as you said, the muscles weaken, and aren't strong enough to push out the poo. Once she got cleaned out, and had an easier time pooping te muscles strengthened and now she's fine (almost). I would beg your Mom to go to a Doc and get a referral to a GI specialist. This is interfering with your life. It sucks enough being a teenager. You should NOT have to deal with this all alone. Get seen, and get a diagnosis. It might be IBS, but maybe not.
Huge hugs, darling.
Formerly HanSolo. IBS, OCD, Bipolar, PTSD times 3.