All the stuff you did when you had a young baby, plan her hens, maid of honour etc, did she definitely expect you to do all that? Or was it more YOUR idea of what you should be doing, even though it wasn't reasonable to expect a new mum to be doing all that? Maybe she didn't want to tell you it wasn't necessary?
To be honest with you, I think it was a bit silly to do all that stuff for her at that stage in your life just "because that's what friends do". Especially the 4 day hen. It's a bit martyr-ish. If she WAS expecting/asking you to do all that stuff I think you should have been assertive and said that while you'd love to come to the wedding, you'd happily let someone else be the maid of honour as you weren't really up to it with being heavily pregnant/or having a new baby, and that it was better to give it to someone else who had the time and energy it needed etc etc. And that you would come for one night to the hen, not 4. I think that would have been the time that you would have found out the type of person she was. A good friend would have understood, let you take more of a back seat, and then you wouldn't have felt resentful all this time and even more hurt at her deciding not to come to your wedding now. A poor friend would have had a bridezilla hissy fit, and then you could have made the decision THEN to back off a bit rather than having this upset now about your own wedding, which seems so much more of a contrast to all the effort you put in for hers.
Personally I find it hard to trust people who constantly do things for others but secretly find it too hard or havent got the time and they resent it. If I were on the receiving end of that "giving" I'd be horrified if I found out that it was being done under duress and not enjoyed. That's not how life is supposed to be. I like to take people in my life at face value, so if someone was organising something for me I would want to be able to trust that they really wanted and were able to do it. I could never enjoy an occasion where my best friend felt exhausted because of it, so I'd rather not have the occasion!
I think honesty is the best policy with "duties" like that, really, where weddings are concerned. Not everybody finds them an incredible honour to be asked. Mostly it's a pain in the arse, let's face it.
Re your own wedding, I'd simply tell her that it was totally understandable her not wanting to be maid of honour so that was quite alright. And that any way she wanted to attend your wedding is fine with you as you value her being there so much so whatever works best for her. I'd also add that if she has a newborn baby at the time of your wedding and feels so overwhelmed that she doesn't want to leave the house then that's also perfectly already with you. I'd just tell her to play it by ear but that the invitation still stands, and it can be an open invitation - she should just see how she feels nearer the time.
Honestly I think more open and honest conversation needs to be going on between the 2 of you. It's like you're both too scared to talk to the other one for fear of offending them. Maybe she knows how much a place at the reception would cost and doesn't want to take up a place which she may not be able to attend? You don't know....