Gluing glass to glass help?

I am looking for some advice on gluing glass to another piece of glass. I had two projects where I needed to glue glass together, one a birdbath the other glass flowers. My glass flowers stayed together for awhile, but just fell apart after 2 months I used E6000 on those. The birdbath I dont believe ever fully cured, I used a clear tub and tile adhesive on that. Do I need to rough up both surfaces before gluing? Also, could the amount of glue I used have been too much, I put a fair amount on.

For glass on glass, you need a specific glass glue that cures with UV light (sun or UV lamp). Not sure which brand I used, but it worked fantastically! Other glass glues without UV curing are disappointing.