(sorry, this is really long... well done and thank you if you manage to read through it all)
Lots of background here and I will try to keep it succinct.
Friend and I have known each other for more than 10 years, since uni days. We were very close for many years and she has been like a big sister to me for the first few years of our friendship. Around 8 years ago she really helped me out by letting me stay at her place for a good week, when I was trying to get rid of an abusive bf. She was both practically and emotionally very supportive at the time and I will always be grateful for that.
The problem is that I think she measures everything she gives and then requires equal amounts of gratitude in return. She always needs to be the one in charge/in the powerful position. This is particularly difficult because my mother has a similar issue and it took me years of therapy to disentangle that toxic relationship.
For example, she has not once in the last 8 years attended my birthday party or got me a present. There is always another birthday, or her sister's show, or a trip abroad (etc.) that is more important. She has sent a card once or twice for Christmas which was lovely.
For my 30th, she said that she couldn't come to my party (she lives a 3 hour drive away and said it wasn't worth just coming for the weekend). Her reasoning was that she'd rather invite me to her place the following weekend, take me out for a meal as a treat, that sort of thing. So when I arrived at her house (I was heavily pregnant at the time but made the effort to travel up on the train), she kicked the weekend off by drinking an entire bottle of Champagne "in my honour" (as I wasn't drinking). We then went food shopping for the dinner she was going to cook for me, and at the till she asked me to pay half the bill. After the dinner, I said that I really fancied going out, sitting in a bar, just being out and doing something special (I had an 18 months old at home and hadn't been away from him since he was born). She didn't like the idea because she said we had "everything we needed" at her house (i.e. cigarettes, weed, alcohol). We did end up going but she was grumpy the whole evening.
I was also going through a bit of an emotional time during this visit, and she just wasn't able to listen to me. We did a lot of talking about her issues and problems (and her family's and friends' problems...), which I am usually very happy to do. On this particular occasion, however, I felt a bit resentful because I had hoped for a "treat" weekend and instead was made to feel like a bit of a burden to her...
On the latest visit, I drove up to see her for two days again. I wasn't pregnant, so was able to at least enjoy the Champagne this time! It was a nicer visit, also because I had adjusted my expectations and wasn't looking to find a listening ear. We talked a lot about her problems again (which was fine, I was happy to listen), and then I initiated a night out. We had a great time dancing, going to a few bars etc. The problem was that she hadn't brought any money with her, so I ended up paying for the cigarettes of which I smoked two and she kept the rest (and which she put in her bag as soon as I had handed over the 10 quid to the cashier 
). I also paid for the entry to the club and the taxi home for both of us. She did have her card so paid for half of the drinks at the bar.
Again, although I had a fun time with her, I ended up feeling a bit resentful and taken advantage of.
The final straw came the next day, when I was back home and realised that I had left my gold necklace at hers. It is a very precious necklace with the names of my kids engraved in it. I asked her if she could post it to me, but she said she was too busy that whole week (week before Christmas, so I understand things are busy). So I asked a friend at work who has family near where she lives, and he said that she could just drop it off at their house ( a ten minute drive). Again, she said she was too busy. So I arranged for the friend to pick up the necklace from her house, but again she said she couldn't guarantee that she would be home at any particular time and she didn't want to commit. She suggested that she would bring me the necklace in person next time she came to visit, around April time. And surely I could wear "some other necklace" until then. I did tell her how precious that particular necklace was.
At that point I thought that maybe our friendship is just not worth it any more. We do have a nice time together, but I always feel that I can't quite be myself around her, and that she is quick to judge, offer "advice" which isn't really wanted and without listening to what I'm trying to say first.
Complicating factor: She has invited me and my family to her wedding this autumn. It's a 6 hour drive for us, two nights' stay in a hotel etc. We're not rich, so this is quite a big deal for us. In addition, due to some complications with her fiance's visa, the wedding might not actually happen in the end, and could be cancelled at a short notice, which means we would lose whatever deposit we would pay on our hotel/airbnb. Given how little effort she has made to come to my birthdays over the past decade or so, I am again feeling unsure if I want to do all of this for her. And I don't want to go to her wedding feeling resentful about her. So I'm considering not going, but I'm aware that this would mean the end of our friendship.
I just don't know what to do. Clearly she is making me feel bad at times, and I'm aware that she is touching a sensitive spot because she can be so similar to my mother in some ways. At the same time, she can be very generous (although her generosity sometimes seems like a bit of an ego trip, if you see what I mean) and when she came to my wedding she filmed large chunks of the day and then put together a beautiful video for us.
She herself comes from a really messed up family, where she was always made to feel like she isn't good enough. She doesn't seem to acknowledge this at all though, and always fiercely defends her mother in particular (who I think is extremely toxic and egocentric). I keep hoping that once she resolves these family issues, a lot of the bitterness and selfishness that I see in her would dissipate and I would be left with the loving and fun friend I know she can also be.
Shall I keep waiting for her to change? Lower my expectations as to what this friendship can offer? Or just count my losses and finish the friendship?
And: should I go to this wedding, or should I not?