No you're not being ridiculous, it's a totally normal reaction IMO.

Yes, he has a right to get on with his life but the speed and past unhappiness must be complicating the grieving process for you (a friend went through something very similar and struggled too). You say your heart broke a little when you found out about the overlap with the gf - I think most people would be feeling this was a betrayal of sorts and feel hurt on her behalf.

You are doing the right thing IMO saying you are happy for him - I don't think telling him your thoughts would help anyone. He will do what he will do.

Allow yourself to feel what you are feeling, it is perfectly understandable, grieve for her in the way that you need to. I am very sorry for your loss.