I know the feeling. A while back I had to discover some new music because I was pretty much only listening to DT at the time.
I'll give you some of the bands I've been listening to along with DT.
If you like A Chnage of Seasons, I would recommend listening to The Great Escape (song) by Seventh Wonder.
If you like Peruvian Skies or The Mirror, I would recommend Schism or Vicarious by Tool.
If you like These Walls or Illumination Theory, listen to Visions by Haken (I
love this song/band)
If you like Space Dye Vest or Vacant, listen to The Raven that Refused to Sing (song) by Steven Wilson.
If you like Beneath the Surface or Surrounded, listen to Great Expectations/Kneel and Disconnenct/Drawing the Line by Porcupine Tree. (Side note, watch Jordan Rudess and Steven Wilson perform Lazarus)
Also, some bands/songs you might like that I can't think of a comparison to at the moment:
Redemption - Threads
Sonata Arctica - Losing My Insanity
Opeth - Black Water Park (Heritage album if you don't like growls or screams)
Circus Maximus - Abyss or Game of Life
Symphony X - Anything from Paradise Lost
Other bands:
Blackfield
Leprous
Cynic
Animals As Leaders
Planet X (founded by Derek Sherinian and Virgil Donati)
Anathema
Rishloo
Beardfish
Bigelf
Transatlantic
Gazpacho
Devin Townsend project
Fates Warning
Flying Colors
Spock's Beard
And finally, listen to other DT member related stuff:
John Petrucci - Suspended Animations
Jordan Rudess - The Road Home
James LaBrie - Impermanent Resonance
Liquid Tension Experiment - Self Titled
I'll add more if I think of some