Lost and hopelessly confused

I'm sure this is not going to be new ground here on this forum but it is certainly new to me and i'm so upset, confused and have no idea where to turn for help.

I've been in a 4yr relationship with a wonderful woman and the first 3 1/2 years have been the best of my life and we are in our late 40s.   Besides being a beautiful, attractive woman she was so appreciative of me and all the little things no other woman has bothered to recognize.  She was always very attentive, highly attracted to me and we had a mutual fondness for each other very quickly which only grew stronger over time.  The communication was very open and forthcoming - she never left me wondering how she felt about me or anything else for that matter.  She did tell me early on that she had Adult ADD and at the time was taking some meds to help her focus at work.  The only signs i really noticed that seem familiar with ADD (from my reading) is her poor organization and follow through with some things.  She always intends to do something and is all excited about it, then it just slips through the cracks.  Occasionally it had something to do with our relationship, but mostly other areas of her life.   I've never felt closer to anybody in my life - we have the same interests, humor, lifestyle, family values etc..

Early on it was clear she wanted to get married.  I'd been previously married (more than once) and it was not that important to me at the time.  The longer our relationship went on, I could tell how important it was for her to feel a commitment and to have a ring to signify it and honestly I think the pressure from everybody wondering when this seemingly perfect couple would tie the knot.. and when she'd have a ring etc...  She'd never been married and i knew she wanted the fairy tale romance.. and i wanted her to have it.  As time went on - i just knew she was it for me, and realized I was letting past mistakes influence my perspective. I knew she was who i want to spend my entire life with and it was silly not to jump on board.  I wanted us to live happily ever after and have her as my wife.    I created a fairly large proposal scheme and asked her ot marry me last Summer.  We don't live together but she was so happy and immediately started planning a wedding for later in the year or early next (2016) and we started talking about moving in together, which would be a relocation for her.  There is one other significant detail i'm leaving out, but it could be very relevant.  She had a third surgery over the holidays for a serious condition she has which left her with chronic pain and some other not-so-fun symptoms.  She lost her job and her prognosis is uncertain. The first two were the begriming of last year (so we had gone through one before our engagement).  During this time she's been on a  variety of medications - some narcotic including (Valium).  She's already weaned herself off everything besides a Valium occasionally - perhaps 3-5x week (that i'm aware). 

Over the past few months i've noticed she has completely stopped talking about our marriage/engagement.  When i bring up moving in together she has a long line of excuses most of which can be attributed to her medical condition and the timing etc..  While some of this is valid - I've told her i just want to start planning, we dont actually need to do anything until she's ready.  Nevertheless it has provoked her in to being defensive, angry and still there is no discussion.  It seems almost taboo to mention we are engaged.. and she's all but stopped referring to it.   The million dollar question is "what has changed, have you had a change of heart, do you want to slow down - have I done something wrong, do you have cold feet"?   I've asked every which way but Sunday.  I get the same answer.  NOTHING has changed. My feelings are the same, i still want to get married etc..etc..  Since i never see any change in behavior or emotions.. ive asked quite a bit and now it just makes her angry that i don't believe her and she is defensive claiming that she still acts the same.. etc..etc..

At first i thought it might be all the medications + pain, however, we'd been through this before last year and I didn't see the same type of behaviors.   This time though, since we are engaged I started wondering if ADD and the "chase" being over had anything to do with what im seeing.  

I feel that I am no longer important. She isn't making any effort (certainly not like the past 3 yrs) to make me feel loved or special.  In other words, i feel like im in a relationship with somebody who looks like the love of my life, sounds like the love of my life.. but it is somebody I've never met before.  

Can anybody provide some insight and thoughts about if/how ADD might explain any of this?  It seems that 3+ years is a long time for "hyper-focus" to have gone on.  From what little I know/read about it - it seems that ADD would have lasted a much shorter time before the excitement started to wane.  I know i can't be THAT exciting to have enjoyed over 3 years of it.  Does anybody think I would have seen a big change before now if the ADD was behind this drastic change?  Is a contributing factor perhaps?  Having a hard time (which is unusual with her) getting her to open up and discuss anything without it getting defensive.  We've always been able to resolve issues rather easily until now, so this is new and i'm at a loss.   

I love this woman with all my heart and feel that my entire future, my hopes and dreams are crumbling right before my eyes and i can't do anything to stop it.  Worse you can imagine at my age I have one eye on the clock watching it tick down.  It doesn't feel like I have "forever" to figure this out and help her (if i can) - help me understand and move on from here.   I know somebody is going to ask me "what if this is just the way it is - what will you do"?   I don't know. I really don't know.   I can tell you that the uncertainty of her status, and this puzzle i feel she has given me to solve is not fun. And I don't think i can do that the rest of my life.  We have to be able to talk about whatever it is - or it's futile.

Thank you in advance