That's quite amazing that spice is the only psychedelic that you have done!
Have you smoked enough to have what is described as a "Break-through"? Do you use a freebase pipe? There really is no other way!
Except maybe to inject it or make ayahuasca
I personally have had very heavy experience's from spice, but never what I would call a bad trip. More enlightening then anything.
When I discovered spice I managed to get a hold of a fair bit and ended up sharing it with about 30 people. Out of these most had a beautiful experience but 3 of them did get quite thrown back by it. Two of them freaked out totally and I had to really work hard as the sitter.
Strange thing though, out of the three, one was a very intelligent but somewhat skeptical/cynical type that had enjoyed other psychedelics, one was a hardened acid head but quite the control freak, and the other was a totally "common" type dude with not much in the department of spiritual or intellectual views and was a user of ket, coke, mdma.
I might add that no-one actually had a technically bad trip as each of the three that freaked were laughing in amazement with minutes of returnig from their trip.
I think the connection is that the more right -brained creative sorts ( everyone else I gave it to) found it more comfortable to let go and enjoy the trip, although a few got "slapped" for being "greedy".

The more left-brained logical types were more lost/scared without there consensual reality to hold on to.
If you want to gain a deeper understanding of the psychedelic state then I might suggest you try some psilocybe mushrooms for a longer experience, although not as intense as dmt. A much better option than Lsd though IMHO.
Whereabouts do you live? You might have fresh shrooms very close to home as its the season.
http://forums.mycotopia.net/wild-mus...d-my-area.html (Which psilocybin-bearing mushrooms grow wild in my area ?)
Be very careful what you pick and eat though, come back and check with us with anything you find