I realized I am an enabler. How do I learn to say no to my addicted loved one?

Good for realizing. You know you have the behavior but can't stop, sound similiar to some people with addictions huh? And they usually need help, well no exception for you, you likely need help and support to stop this enabling behavior. Recommend trying alanon and/or seeking professional help with a counselor that can help you in setting healthy boundries. The addicted may or may not change, but you'll be more sane.
Enabling. Try to practice setting reasonalbe limits as when you tolerate inapropriate behavior you lose self respect and you are not helping your partner. Try to attend an al-anon meeting as the focus is on correcting enabling behavior. Also check your guilt as this emotion can make you tolerate behavior you should limit. If you cannot make these changes consult a psychologist in your area. Best wishes.