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Im not a marriage counselor nor am I married so I cant say for a fact what would make someone unsuited as a spouse but there are certain basic traits that cause most marriage problems.
Being selfish or uncompromising
Insensitive to others
Anger issues
Control issues
Low self esteem (depends how low) or lack of confidence in self an abilities
Overly critical or judgmental
Unforgiving (bearing grudges)
Aloofness or closed emotionally
Unable to admit wrong or apologize
Disloyal or untrustworthy
I could go on, but since you were once married you can answer a lot better than I can and Im sure you know that many of the things I mentioned can be major factors in marriage so if any has a problem in the above areas it must be worked on before marriage.