Husband left 3 months ago, wants a divorce, still pays for everything, no children. Been married for 18 years. Found out he might possibly have a girlfriend.

No its not abandonment. But even if it was, what does it matter? Do you want a divorce? If so, you file for divorce. Depending on your location (ANY question on law needs to include your general locale as laws vary by area) you may be able to use abandonment as grounds for the divorce or you may not need to. Most US states have "no-fault" divorce so you can get a divorce without any grounds.