I certainly hope you haven't drunk that bottle of oramorph in the time you posted and I logged on this morning, OP!
Chronic pain can be extremely debilitating and the depression it causes can easily turn to despair. What have you been doing to help yourself? Have you put yourself forward to join an EPP //www.nhs.uk/NHSEngland/AboutNHSservices/doctors/Pages/expert-patients-programme.aspx and have you posted on this thread for advice and virtual support from other sufferers? //www.mumsnet.com/Talk/general_health/2469596-Chronic-Fatigue-Syndrome-ME-Fibromyalgia-for-support
Medical research is at a particularly promising stage and stemcell treatment offers the prospect of curing many ills. Have you searched online to see whether any clinical trials are being undertaken for any of the illnesses you have?
Do your GP and other health professionals know how down you feel? Are you taking anti-depressants? If so, perhaps it's time you had an appraisal of the medication you're on with a view to finding a more effective drug that will lift your mood.
The devastation that suicide causes to those left behind can't easily be put into words. Their lives are never the same and they are condemned to forever wonder what they did wrong, why they didn't pick up on the 'signs', whether they could have done anything differently, and why their loved ones didn't love them enough to stay with them. Contrary to your opinion, none of them will ever think they are better off without their loved ones in their lives.
Suicide is an opt out clause for us all, but imagine how you'd feel if you successfully despatched yourself to the afterlife only to find that you had to come back and live this one over again in order to attain a different outcome? I'm not saying this will happen, but it's as possible as any other theories that are put forward about what happens to us after death and it's certainly enough to deter me from topping myself.
I am so sorry your life hasn't turned out as you'd hoped but it is still worth living and, while he would no doubt far prefer you to be in good health for your own sake, I'm sure your wonderful fiance has no complaints about the hand fate has dealt him in having you as his loving and loyal fiancee.
Miracles happen every day. I hope that one is in the making for you now and that you'll find the strength of mind to stick around to receive it. In the meantime, reach out to others and ask for advice and practical support to help you bring about improvements to your quality of life that will give you optimism for the future.