Will I go to hell for not paying a bet?

Yes. You are doomed. Unless you repay the debt twice over.

The Bible does tell us to pay back our debts. Consider principles that apply to the borrower. The apostle Paul exhorted Christians not to be “owing anybody a single thing, except to love one another.” (Romans 13:8) While Paul was stating a broad principle here, his advice can certainly be taken as a caution against incurring debt. Sometimes it is better to go without than to owe money to someone else.  Proverbs 22:7 explains that “the borrower is servant to the man doing the lending.” The borrower must realize that until the money is paid back, he is under obligation. In principle, his resources are not entirely his own. Paying his debt in harmony with the terms agreed upon must have a high priority in his life, or difficulties will likely arise.
Hell is just the common grave. The Bible does not teach of a burning hell, so, no you will not burn forever.