Unfortunately, nothing. Your boyfriend's promise is barred by the four-year statute of limitations. In addition, it became an asset of the bankruptcy estate, and the trustee became the legal owner. Unless it was transferred back to you at some point during or after the bankruptcy, he does not owe you the money.
Some lessons have to be learned the hard way and Shakespeare admonished "never a borrower nor a lender be." He's a jerk, but at least you didn't marry him & I hope you didn't have children with him! Let it go so better things can come into your life. Hope this perspective helps!
What a jerk!
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You have 2 issues. First is the Statute of Limitations. The 4 year statute runs from the date of default, which might be the payment date the last time when he promised to pay you. If the written contract does not contain a payment date, it might be construed as being payable "on demand." The second issue is the BK. The BK did not absolve him from his obligation to you. However, depending upon how the paperwork was done, it became an asset of the BK estate (administered by the Trustee), not your personal asset. If the Trustee informally abandoned this asset, you might have standing to sue. You should have a BK lawyer review the specifics of your BK to determine this.