Weddingbee boards

So I kind of did this to myself, but heres the story.  Any advice is appreciated.

"Tina" and I were close in college.  We were there for one another and had a fun, trusting friendship.  After college we kept in touch, and though we have drifted apart somewhat over the years, I wanted to honor her friendship to my by asking her to be my bridesmaid.  She excitedly accepted all was good in the world. 

Then reality kind of hit me.  Tina is an amazing, creative, caring person who has a mental illness.  Her mental illness can make her a dark, brooding, unreliable, untrusting, chaotic person at times.  I have noticed a pattern in the times I have met with her since college that is begining to concern me. 

Tina lives in another state but sometimes visits the state that I live in to see family/friends/reunions/etc.  I always make a point of meeting up with her when she comes to my state and will go to wherever she is so that we can visit.  Whenever we try to make a plan to visit, I will almost always get a call/e-mail/text a few weeks before we are supposed to meet telling me that she has cancelled the trip due to finances.  I understand that money is tight and I have always been understanding of this.  A few days later, she will contact me saying "Yay!  I'm coming", so I change my plans again to see her.  In the past, this will go back and forth and about 50% of the time she acctualy comes.  The last time we saw each other was for a reunion, during which we made plans to meet each other to shop for a concert we were in.  When I called her to tell her I was on my way to meet her she informed me that she had had some form of drama with someone and had already gone shopping.  Its always a crapshoot as to whether or not she will let me down by changing plans unexpectedly. 

Well, we are supposed to meet in a few weeks and we have already gone throught the "can't come, oh wait, yes I can" cycle once.  I am, frankly, feeling a little tired of the game and I cannot help but begin to worry about my wedding.  Will she pull this at my wedding?  Will some sort of "crisis" occur on the day of the wedding?  I don't know.  I though she was doing better with her mental illness, but this pattern is disturbing to me.  It makes me feel as though she manipulates me (and others) with her behavoir and I am not sure of how to proceed. 

Any advice?  Does anyone else have bridesmaids with emotional issues that are negatively affecting them?  I'm feeling torn and manipulated. 

MissAsB, Thanks for the reply :)  She is not currently medicated or seeing a therapist due to finances (her illness is Borderline Personality Disorder, which is extremely complicated and difficult to treat) and while she is trying to work out her issues on her own, it can get sticky.  I think I do need to talk to her, just a have a heart to heart and let her know how I'm feeling.  Part of me is afraid of setting her off, but I cannot have a fake relationship with her just to avoid conflict.