I agree that you NEED to get your guitar back and as soon as possible -- and yes, you should take someone with you when you pick it up.
I don't agree that you should say something about NOT caring about his life. You don't want to ruffle his feathers and get him mad, or else you may NOT get your guitar back! He may just decide never to be home or never answer the phone when it's you, etc., when you're trying to set up a time to come get it.
I wouldn't say anything about the quality of his life. Just talk about WHEN you can get your guitar back -- you can always "let him have it" after you have the guitar in your possession.
Dearheart, this man is TOXIC to you. Once a cheater, ALWAYS a cheater. You've seen it happen on more than one occasion. Do you want to put yourself thru it even more?
You CANNOT even think about getting back with this guy --- he is not WORTHY of you!!! Find someone who deserves you! Find someone who will love you as much or MORE than you love him! You deserve the best! God bless and please take care! Hugs, Lee