Does Graystripe still love Silverstream

This is a difficult question to answer. Silverstream was Graystripe's first true love. He never states in the book who he loves more, but he does express his feelings to Millie as follows: "I'll never stop loving Silverstream. But she's with StarClan now. And since I've met you... I'm not lonely anymore." (The Lost Warrior, 94)

He loves Silverstream more,

She was his true mate.

the only reason why he expresses his feelings with

mille is because he gets to see her always.

With Silverstream he never got to be with her for long.

From: Poppyfrost

It's obvious he loves Silverstream more. It was fate even though they were from two different clans. But when she died, he had to find someone else. Millie was perfect for that spot. They both love each other but Silverstream is always there.