Let me just update this with my own experience. The police are probably not questioning you unless they already have some evidence against you or somebody else. If the evidence is against somebody else, follow what the Original Poster said.
If they have evidence against you -- and they will make it clear that they do, because usually they want your cooperation with a larger investigation -- you are screwed. In that case, do as they say. Tell them the information that they want (they will be checking your information against others that they confront, so don't bullshit them).
There is a fine line here. Technically, discussing your past drug use/dealing can in principle get you in more trouble, even if they don't have physical evidence of it. For example, if they intercepted 100 tabs of Ecstasy that was being mailed to your house, and you admit to dealing 100 tabs each of the last 12 months, they can in principle charge you for all of that. But if they are in your living room asking you about it, and they haven't arrested you, they want to know your source. They have bigger game in mind.
The first thing you should realize is that there are no heroes in drug use or dealing. Nobody has your back. If you don't cooperate, somebody else will, and they will only screw you in the process. It's a game theory problem. Your best option is to cooperate. Tell them about your source(s). Tell them everything that you know. With your cooperation, the vast majority of the time, they won't charge you for the others things that you admit to (like the past dealing of E in the example above).
It's a heart-wrenching call for many people, but that's how it goes. If you don't do it, somebody else will. You're a lot better off if you do it.
So if you're being questioned, and you have a good idea that law enforcement already has evidence against you, COOPERATE. I'm saying this as somehow who has been federally prosecuted, and who has had more experience with state cops, DEA, and federal prosecutors than I would wish on anybody. And I did it the wrong way. Minimize the damage to yourself.
Yeah, the system sucks. I wish drugs were legal too. We don't live in that world. In some countries, drug users / dealers are executed. Take comfort in the fact that you will not face that fate. You will probably be negotiating between certain numbers of months in prison, or probation. There's always probation.
Also realize that nobody is really your friend in the drug world. It's full of scammers and reverse scammers, and that's all the more true in the online world. I've seen one reputable source get taken for $7000 only to turn around and take another guy for $9000. He went from reputable source to scammer in the blink of an eye.
Look out for yourself, and do what you have to do to minimize the damages to yourself.
This might be blasphemy to many people, but you won't understand until you're there. Everybody is a hero until they are on the chopping block.