Surely the dentist will start treatment and get you out of pain, let you pay something and let you pay at the end of the month?
Also, in case it helps, I was the world's worst dental patient. I used to hyperventilate, sob like a baby etc etc, didn't go for years and then only in extreme pain. I finally found a dentist who built up my trust, she was so understanding, never made me feel like a wally, and eventually when I had a crown and root canal she said I was the calmest patient she'd had as it was particularly long and difficult.
I looked into breathing techniques, how to calm myself, transport myself elsewhere while it was going on. I have issues with someone being an inch away from my face (!), it makes me claustrophobic and panicky but I shut my eyes and dream about stuff, holidays, mountains, oceans whatever, I am just not in that room, it is not happening to me. Kind of hynotise myself if you see what I mean. Sounds a bit mental but I didn't want my teeth in a jar by my bedside by age 40 and I had to get over it for my children too, didn't want them to take on my fears.
ANYWAY, back to you....!!! Don't suffer this, get some help from somewhere, beg to defer payment and deep breaths.