Would you tell her you love her, even when she's going to married soon?

I would tell her in person. Sit down with her over coffee or something. Ask about wedding plans and how things are going. Then saying it kind of jokingly, something like, " I've known you for a long time, and I can hardly believe you're getting married. We're getting so old. And I never expected things to end up like this, you know? I've had the biggest crush on you for as long as I can remember and I feel like we're breaking up now because you're getting married. I'm so sorry I never told you until now. I just didn't realize that I would run out of time. You always seemed to be in a relationship every time I got the nerve up to tell you and now... now I want you to be happy, but you have to know that there's a part of me that can't let you go. I want to support you and do what friends are supposed to do, but I'm in love with you and there's nothing I can do about it, except tell you. I'm sorry if this complicates things- that wasn't my intention. I just couldn't stand here and lie to you anymore; pretend that I didn't feel anything when I do." Something like that- where you ease her into it, you provide a reasoning for not mentioning it sooner, and let her know that you're not trying to ruin her wedding or your current friendship- you just have to be honest. Good luck.

"she is always with someone else"

And what exactly does that tell you about her interest in you? Surely you don't think she's just been filling her time with these other men while she waited for you to speak up, or that she'd agree to marry one of them if that were the case? If confessing is the only way for you to accept this is all in your head and finally move on, then so be it, but be prepared to do that.

You had your chance - for years and years. You had your chance when she started the dating this guy. But once she accepts his ring, she is off the market and no longer taking new offers.

You had plenty of chances, but you didn't take them. You even wrote a letter, but didn't send it. Deep down, you know this isn't right, and you know it's all about your obsession, and not any chance that she will reciprocate your feelings. Don't be selfish and mar her happiness.

There are 1000's of love stories which are paused due to lack of opportunity or courage. I think, its too late to express your love. Just take care of yourself, find a new partner and dont delay. Time will heal everything, she will remain in your heart.

If you respect her leave her be. All these years you had your chance and didn't take action. If you want to tell her, then you better be damn sure it is worth screwing up her plans to marry another guy.

If you have had feelings for her "for years" why would you tell her now? You missed your shot. She is marrying someone else, and regardless of what you say, this will not end well for you.

This is just like friends with ross and rachel lol

Yes. She needs to know. She has to know her options, shop around a little.

yes you should let her know before she gets married

I do