A promissory note is valid regardless of the type or quality of pen and paper used, but if your ex-boyfriend forced you to sign the Note, you may have been legally under "duress" such that the contract should not be enforced. His subsequent conduct may or may not be admissible, but if he threatened you physically when you made the Note, I think you have a viable duress defense.
Needless to say, you should consult an experienced contract litigation attorney.
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