I'm the maid of honor & i don't have a choice

Hi all, I'm going to begin with backstory/context:
My "best friend" and I are childhood friends for over 15 years. We both grew up in unstable households, and we relied on each other for emotional support. For the longest, I viewed her as a beautiful person inside and out. I could never fathom her hurting me, but that has changed.

Fast forward to the past year. She (F25) always has struggled with money, and her fiance (M28) is the breadwinner. I wanted to help her out financially by moving in together and being roommates. So my boyfriend (M27) and I (F24) moved in with them in a two bedroom/two bath apartment. We all knew it was a temporary measure, and we would only stay for a year and then move out.
Well, her fiance decided to pop the question, and she wants to plan a wedding in less than 10 months. EVERYONE insisted she waits a year at least (for financial reasons), but she did not want that. She made me the maid of honor without asking me because I'm her only friend. There are no other bridesmaids. She barely likes her mom, yet her mom is contributing thousands of dollars to the wedding. Also, I'm a bridesmaid for my other friends wedding one month before hers! She didn't consider that. My other friend has been planning this wedding for two years now.
Personally, I don't like the fiance. He is a slob, he doesn't stand up for himself, and he has severe social anxiety where he can't leave the house without puking multiple times. He let's my friend do whatever she wants. It's such an unhealthy relationship, and the thought of them having children insights literal fear in me.
Believe it or not, I was going to suck it all up. I was going to bite my tongue and say, "if you're happy, I'm happy." Well, that changed after she started to critique my relationship and say we need "couples counseling". My boyfriend has been, by far, the healthiest relationship I've ever been in, and my therapist agrees.
**TW: SUICIDAL**I suffer from seasonal depression; and I tried talking to her about my issues... and she said that she has her own problems to deal with and she can't handle mine. She asked me to not talk to her anymore about my problems because it raises her anxiety. My emotions make her feel like she's trapped in her childhood home. Having to suppress my feelings, after 7 years I began to feel suicidal. My boyfriend was (rightfully) upset about other things, and I couldn't talk to him. I called my friend as a last resort, and she proceeded to yell at me. She didn't even know I was suicidal, she was just mad that I was making her anxiety rise. After that phone call, she yelled at her finance for over an hour. I made her that angry.
Since then, I haven't talked to her about ANY of my problems. A few days after the incident, she brought it up. Idk why. So I told her, bluntly, I was suicidal during that phone call. She stayed quiet, then changed the subject. My boyfriend saw her reaction, and he honestly doesn't think she's a good friend to me. I've talked to three other friends and they all said if I ever feel that way again, to call them instead. I hope I don't.
It brings tears to my eyes that I can't depend on my lifelong friend during rough times. She only wants me around when it benefits her... aka the wedding and paying half the rent.
Now, I don't want to be a part of the wedding because of two reasons: I think their relationship is supremely toxic & codependent, and she is taking advantage me in a similar way.
I honestly never considered the idea that a friend could be toxic, especially her. It hurts so much. But I'm scared to even talk to her about it because she gets defensive/argumentative... and we live together. It will be dreadful. The wedding is one week before our lease ends. I feel like there's no way out & I'm trapped in agreeing to something that I never even had a choice to.

I don't know what I'm looking for here, but honestly I'm too fearful of what could happen. I'm afraid I might have a mental breakdown come wedding day.