It definitely is. He's VERY focused on the past. He spends all that time in uthenera, wakes and spends so much time in The Fade, a place of impressions, memories, the past. He focuses a lot on the past, mistakes, turns his own regrets over in his head a lot. Thanks to Cole and other dialogues with Solas, we get this. He talks a lot about things that 'still remain' or what was lost.
His discussions with Varric illustrate this very well. Varric is a pick up the pieces, make a life, do the best you can sort of person....a degree of optimism. Solas asks him about the fall of the dwarves and what was lost and the story of the man left alone on the island after his wife died in childbirth that Solas tells is very revealing. Solas' POV is more pessimistic, counting losses. Varric talks about the guy making a life, even if he was alone.
Solas cannot let go. His thoughts are frequently in the past, sometimes they're a certain melancholy. His banter with Cole about It's a Wonderful Life - Cole's take is the traditional take, the angel showing George that he had much to live for and prevented his suicide. Solas' take was that it held him as he jumped and died and comforted him'.
One thing you, especially as a romanced Lavellan, give him is a little bit of hope. Other companions also help teach him ways to expand his perspective and assumptions too.