Don't be so hard on yourself. I have crippling health anxiety at the moment, to the extent where I can't watch health stuff on TV in case I 'get' a new disease. Every little ache and pain is a potentially deadly disease.
The only cure for me is keeping busy. I left a full time job two years ago and thought semi-retirement would be bliss, but in fact I have really missed regular contact with a known bunch of people and being solitary and working from home has greatly fuelled my anxiety. My GP put me in touch with Let's Talk and I got a telephone assessment when CBT was recommended but I am still waiting so have booked private appointment.
Anyway - this isn't about me, except in so far as it suggests you are not mad and you are not alone. You will visit your friend and you will be supportive, but accept that you also need help. Go to you GP soon, get yourself checked and get some talking therapy booked. Even if your lump is nothing serious, which is highly likely, I would suggest you deal with the health anxiety. I thought mine would go if I could stop stressing about my poor gums, but if I have a good dentist visit I just find something else to feed my health anxiety monster.
Good luck, be kinder to yourself. Keep busy, be with people and take som exercise. I have started talking to my anxiety and mocking it (actually this suggests I am mad!) but it helps when I tell it to f@@@ off!