Is "grimace" the word to use here? In this first paragraph, you're talking about an unauthentically happy face, and grimace is an expression of pain... so I don't think it really fits here.
Kep, someone told me you're not exactly a poet, so what's going on here? Because this is NOT bad.
I'll be honest, sure I think this could use a little polishing, but your expressions of emotion are pretty good. I also like how it progresses from a expression of your feelings to the description of the scene of the marriage.
"I lost a lover but gained a friend." Even though this sounds a little cliche to me, I still like it a lot, and I can't quite explain why.
"Time has passed, but nothing has changed,
There's a ring on your finger, so I guess it has."
These two lines seem a little awkward to me. I think perhaps you might want to try expressing that you THOUGHT things would never change, but now that there's this ring on her finger, they have to.
Finally, there's something I didn't quite understand here. You're the best man? I don't know much about weddings, but I thought that it's generally the groom that decides the best man. I also thought that it's the bride's father that walks her down the aisle... hmm... so that part left me a little confused.
Apart from that, I'm going to make the common suggestions... try to maintain a more or less fixed syllable count. If you don't like that, then make it a little more freestyle, use a lot more poetic devices (eg: alliterations, metaphors, etc).
Best of luck.