United Airlines flights from San Francisco to Cancún (SFO - CUN)

i missed the cutoff for luggage by 1 minute in eugene. the woman told me i have to bring my big suitcase with me and check it on the plane but i would be fine . There were several others and getting through was long and by the time we got to the gate they had just shut it . i was out on the next flight from eugene to san francisco but could not get on a flight from san francisco to san diego until late in the afternoon. i had to be in mexico to give a talk and they said that there would be no room on the first connecting flight. So i purchased a ticket on line to be sure I could get to my talk. I went to the United Agent in Eugene behind the counter and explained what i had done and she promised me she would be sure my suitcase made the new flight . It did not . i am on my way to Tecate Mexico and my suitcase with all my clothes and information for my talk is in san francisco . She was certain and said for sure the suitcase would be with me and I am very distressed -I used 55,000 miles to make sure i was on time and now i don’t have my suitcase abs necessary things and no one seems to know how to get it to my hotel .