I have been apply the advice on this site since my ex left for the third time a year and a half ago. We were together for 18 years. He met someone new a month after he left and moved in with her. I don't know how to keep going without him. He seems happier now.

"He seems happier now." - New relationships are exciting. It's not a reflection on you.

"I don't know how to keep going without him." What are your choices? Let this totally consume you where you aren't able to function at all in your daily life OR realize that people fall out of love, breakup, and people survive. You are in total control of how you react to this. Keep yourself busy, go out with friends, and keep your life moving forward.

Never allow one person to control your happiness. You control it!

Your ex left for the third time? I wouldn't have given him another chance after the second time!
You can move on with your life, as I have done. I remember every girl I ever dated or had as a girlfriend, and that was many years ago. Meet some new people. Be honest and respectful, and you will eventually meet someone who really likes you and you him. Don't give up; never give up. It is hard to get over someone, and you will never forget him, but you can move on. Start talking with others, and pretty soon you will feel better. Good luck.