I put the Preventic tick collar on my 10 month old Havanese and almost immediately, he became a lethargic slow moving depressed dog from a very happy bouncy guy that he was before. One week into this worrisome behavior, I read in the Preventic box that it could cause depression. Has anyone experienced this before> Can the collar be the reason for the severe change in personality of my dog?

He is having a reaction to the collar. I would remove it right now and contact your vet as soon as possible. I am not a vet it is just my opinion

Yes, remove the collar now! They are generally safe, but this is a very small dog and may be too much for his system. Be sure to put it somewhere that he cannot chew on it; ingestion of the active ingredient in Preventic can cause toxicity.
Call the vet to see if you need to do anything else besides get rid of the source of the problem.