can i get the other 50% of my retirement back

Property division is final as of the date of the divorce; under Wisconsin law, pensions have been traditionally treated as property, not income, with some limited exceptions. This is true whether the pension at the time of the divorce is an expected future benefit, or it is a plan in pay status. If maintenance (spousal support) was held open at the time of the divorce, it may be possible to go back and ask for maintenance against your ex wife. Another factor in not being able to pursue support over 20 years later is arguably that there is a 20 year limitation in Wisconsin in enforcing civil judgments; while there is not a specific case on the issue, recent case law suggests the same statute of limitations on civil judgments, may also apply to enforcing divorce judgments after 20 years.