Braces Needed? Or Can a Retainer Fix my Teeth? (photo)

I've had braces before. But I didn't really wear my retainer. It's been about 4 years since I've had my braces off. I still have my original retainer. It's the clear kind..kinda resembles invisalign. I tried putting it in for my top teeth. It kinda goes in. But not all the way completely. I am able to wear it without it popping off. Will this help my teeth to get them back the way they were? Or would any other type of retainer work? I really don't want to get braces again.