Now, I know what you're thinking, "Why didn't Edward make up his mind on who he loved?"
Well, simply was because he didn't know who to love, which path to take, who to trust. He'd been abused and hurt so much during and by his past that he was utterly depressed and just wanted comfort.
He thought what he had with Vanillawish was love, but it was too late when he realised she wasn't right for him. She was obsessive, the user type (no, not a Wikia User, the type of person that uses others), and never let him do what he wanted. Although he loved their kits, he couldn't be with her.
With Silivia, his heart was still so broken that he had decided to leave the clan that loved him, that he was trying hard to feel the same way for her that she loved him. Like Vanillawish, it didn't work out long.
He became mates with Raindrop while he was with Silivia. He was already planning to tell Silivia he didn't love her but he learned that Sili was expecting kits. He didn't want to break her heart, so he didn't tell. He truely loved Raindrop, and his troubles were vanquished when she became part of his life.
Now, you can call Edward cruel for taking so many mates and such, but remember,
"You have to kiss a lot of frogs to find your prince." Or in this case, princess.