When will this cycle stop?!

Im back after falling off the wagon. Its my "new" 1 and 1/2 days of being off a 12-15 day habit of Norcos and this *****! :( I was clean this time for 18 days! 18 days! Ughhh im so disappointed in myself :( This will be the third time ive TRIED to get clean by choice and not just cause I ran out. The first time I was clean for 4 months and I slipped for another year and a half. This time its only been 4 weeks that i relapsed but long enough. I was doing great, actually starting to feel like I was getting my life back and then I got the call that changed all of that. My mom whos only 64, was diagnosed with stage 4 terminal lung cancer. I reached for the only thing I knew that gave me comfort (obviously my addiction was still rotting my brain) Whats going to keep me from looking for that "friend" again when things get rough??  I feel that after 4 months and again with 18 days of being sober and still relapsing that I will just never succeed!! Im so frustrasted with myself right now that I just cant even look in the mirror. I know people have told me that relapse is normal but is it really?? I feel like im alone in the world right now.....