:( didn't want to let you go unanswered. I wish I had some wise words to make you feel better. It is shit when you're single and surrounded by happy people in relationships.
Two things strike me - one, you need strategies for getting through the day. Two, you need strategies for improving the quality of your life and your self image.
For the day itself, have a lovely pampering bath, de hair all appropriate areas (where is your business entirely!), exfoliate, face pack, moisturise, do finger and toe nails. Spend all Friday evening looking after yourself. Saturday, get up in plenty of time and have a shower/bath, do hair and makeup and have something nice for brunch to keep you going.
Focus on the positives of the day - your friend is happy, she loves you enough to choose you over everyone else to be her MOH, and there is (presumably) free food! 
I find that in these situations if you just stick a smile on and force yourself to appear as though you are having fun, eventually you start to actually have some. Try to think "how would I want to be in this situation?" and then act like that. Think of a few ideas of questions to ask people you are introduced to - how do they know the bride/groom, are they married/do they have DCs? what do they do? Complement them on their outfits, ask where they bought them - all stuff that people will enjoy responding to. You will get through the day, and it will mean a lot for your friend to have you there.
On a broader scale, you need to think about why you hate your life and what you can do about it. But just get through the weekend first, and be kind to yourself.