Probably the easiest way to think about this is as follows: Have means experienced, the act of doing something Had is a verb that can be substituted for most any other verb if paired with the object of the verb. I have eaten dinner. I have had dinner. I ate dinner. All mean the same thing. The first two are past perfect tenses, the last is past tense. There are instances where using "have had" in spoken conversation with the right intonation can imply that you did something before, but you would not want to do it again. In your last examples, I had it before but now I lost it, and I've had it before but now I lost it, you say you usually go with the second one. Well, that does not make sense to me. It seems to me that what you are saying is "It was in my possession but I lost it." If so, then in this case, "had" is not being used as a substitute verb, rather it is used as a stand alone verb meaning to possess. Using "I have" or "I've" preceding had just does not make sense to me. Also, the "now" while not wrong is superfluous and therefore not needed. "I had it before but I lost it." seems to be the right sentence.

"I had it before but now I lost it" vs "I've had it before but now I lost it" Neither is correct. I had ONE before, but now I've lost it. This one is correct. The past perfect is used when you talk about "further in the past". In context, you always have some reference point in the past, and you use the past perfect when referring to further in the past. Example: It was raining this morning. I hadn't had a shower, yet. So I made the bed and took a shower.

When you want to use an earlier event to describe a later event/situation, you use a perfect tense. I've eaten... - describes now! You ate earlier, and that is the reason you are not hungry now. You can also use this form to talk about experiences relevant to the current conversation. I'd eaten... - describes a later situation which is still in the past. It give a background or reasons for the simple past action: I didn't eat any pizza (simple past) because I had eaten earlier (past perfect - the reason for your lack of hunger). Simple past and past perfect work together.