Struggling with 'Diagnosis'

I'm not sure if this is the right community for someone in my situation but I have to try. Last week I was 'tentatively' diagnosed with IBS-C and I don't know what to do with this.

I've had mild IBS-like symptoms in short bursts since I was about 14. They started at a similar time to my first menstruation so we always assumed it was just that. I have a generally pretty good pain tolerance so I was able to just take some painkillers and get on with my life relatively unaffected. Then last month I became suddenly very unwell. By the third day of this sickness I was in near constant cramping agony, completely constipated, faint, nauseous, unable to eat and extremely fatigued. I saw a nurse that same day who did some exams and tests and told me to go to A+E if it got any worse. By the 9th day, I had to spend 8 hours in A+E. I was still constipated, in agony, unable to walk and barely able to stand with no appetite and pains going up into my ribs. Whilst there, they did blood tests and ruled out any issues with my thyroid, kidney and liver also saying they've ruled out all inflammatory conditions such as Crohn's and even claim to have done a celiac screening (though my GP refutes this). I spent the last 5 weeks on Movicol with my symptoms not improving waiting for an appointment with my GP. I finally had that appointment last week. He said it was impossible to rule out celiac at this stage and asked for more bloods, but since I'm a fainter with veins prone to collapse I decided against this. I mentioned Diverticulitis, a condition that both my mother and her mother had diagnosed when they were considered 'too young' for it. He said he could rule this out without trying anything because if I had it I'd be, and I quote, 'actually unwell'. He gave me my tentative diagnosis of IBS-C which I am already skeptical about because it has now been almost 7 weeks without a single 'good day', my brother has had IBS for years and never had an episode this long. He said he has written me a prescription for Fybogel, but my local pharmacy have confirmed they can't currently stock this and are looking into alternatives since my Movicol runs out tomorrow. He's also placed me on a super restrictive diet - gluten free, lactose free, meat free, low salt etc. He said if this diet doesn't improve my symptoms in 3 weeks to come back and he might consider referring me to a gastro specialist. He also told me to go to therapy and I know that's a pretty standard part of diagnosing IBS but for that to be his first recommendation when he admits he isn't 100% certain of my diagnosis feels strange to me.

I don't know if I'm looking for advice or just understanding. I'm 20 years old, living with a single parent, I am a partial care-giver to a sibling and I can't pull my weight with my family anymore, I get so tired. It has driven rifts between me and my friends because I am near enough house-bound and unable to interact. This restriction diet is making me feel even worse, I am constantly nauseous. My mood is low and I feel like I'm not being taken seriously. i just want my life back.