I don't think psychedelics are generally known for controlling pain although I've read of people experiencing unexpected effects like that. Personally I have never found them to control pain.
But anyway that wasn't really your question... psychedelics are generally not relaxing. I have felt very relaxed during trips, but it's not a given at all. Psychedelics basically make your body feel really strange (this can be bad or good), and they amplify the emotions you're feeling, and make your thoughts a lot more intense. This can produce good and bad experiences. They're not like opiates where whenever you take them, you feel a certain way. If you're actually feeling withdrawal, psychedelics are not a good idea because you'll feel the withdrawal more intensely and it's a lot easier to panic or feel anxiety, if you're feeling those things already. psychedelics make everyday tasks seem brand new, so at least when you first start tripping, you can't really function normally at all. You might spend 10 minutes staring at a leaf or something because of how beautiful it is, or find yourself completely unable to figure out how to interact with other people socially, or what facial expression you're supposed to have. Psychedelics are about the last drugs you want to take if you hope to be able to function normally.
Psychedelics are talked about as useful against addiction because of a single, powerful trip sometimes really helping people to change their perspective and help you to not feel the need to take opiates anymore. They don't help with withdrawal, and aren't something to take daily, or even 2 days in a row. Psychedelics should only be taken once per week at most, and even that rate is too much for more than a few weeks. Every 2 weeks is necessary just to reset the tolerance from one time.
Also, psychedelics have the potential to cause traumatic experiences, especially if you're unable to deal with things the trip may bring up. Sometimes people end up with lasting anxiety or even feel like they've gone crazy or something. It's the same reason why people who go to war can end up with PTSD and it really makes their lives suffer even though it's just in their head. Very traumatic experiences can cause this to happen to some people. I'm not saying it would happen to you, but being in opiate withdrawal is going to make it more likely.
The best and worst drug experiences of my life have been on psychedelics. Well, I wouldn't say the worst, but the most unpleasant/frightening experiences have been on psychedelics for sure.