Summersoul I am so sorry your friend has behaved so badly. You say she is depressed, is this a long term thing or is it recent?
Does she live abroad or is she local to you or far away but in the same country?
I think in your shoes I would write a letter, card or email and simply say.
"We have a longstanding friendship, which I really value. The last year/6 months/ 3 months/whatever has been tough, I found the way things panned out with your wedding very difficult. I spent a lot of money to be there for you on your big day and feel you let me down ignored me whatever.... only go into details if you feel it is really necessary (I am not sure it is at this stage, although it may come out later).
However, our friendship goes back so far I really want to see if we can regain that friendship we once had. I really miss you. Can we meet for a day at a spa/lunch/dinner whatever?"
Unless she lives really far away I would not go for an overnight.
You may not get an apology. Decide beforehand if this is something you need. Or whether burying the hatchet will be enough.
If your friend does not take your olive branch then I would leave it, but leave the door open, if/when, children come along your friend may want to share with you again.
I think part of the thing to learn is to not spend money or agree to things you are not happy with.
Is that what happened? I am guessing you had no idea how much it would all cost at the start (or felt you had to say yes) and maybe it snowballed.
It's very hard to pull out of things when they have started but it is important to remember that you can always say no, or not now, or not anymore.
You can always get costs/expectations at the start of things and if the goal posts move you can be free to say "Sorry that really changes things, I cannot now do that as I cannot afford XYZ."
Sorry if I am misreading it, but it sounds like the thing spiraled in terms of cost or expectations and maybe this was the case for your friend too. And both of you felt unable to say to the person who was upping the ante/making changes etc - in your case it was her but maybe someone (fiance/Mum or Dad or Inlaws-to-be) escalated costs etc. Just a thought. I may be wrong.