Seth Peterson -- Pregnant Wife Files for Divorce ... He Left Me for Some 22-Year-Old

"Burn Notice" star Seth Peterson's a reckless druggie who abandoned his pregnant wife for a 22-year-old woman ... so claims his pregnant wife ... and now, shocker, she's filing for divorce.

Kylee Cochran filed the docs in L.A. citing irreconcilable differences. She says 43-year-old Seth bailed on their 12-year marriage in September to shack up with his 22-year-old side piece, while his wife was 3 months pregnant with their third child.

Kylee is now 7 months pregnant and wants a judge to give her full physical and legal custody of their kids, including their 5-year-old son and 8-year-old daughter. She claims Seth is already okay with this, and agreed to give up all custodial rights.

Kylee says she's open to giving Seth visitation, but only under supervised conditions, given his alleged past drug use.

And she wants spousal support -- if she's telling the truth, it's only fair.

Calls to Seth weren't returned.