How do you end a friendship in an acceptable manner?

I have been autistic nearly 50 years and was the typical severely autistic child. I was diagnosed with HIgh Functioning Autism (Kanners) when they were seen as seperate things years ago. I think very literally nad very black and white and have spent my life dissociating to cope with getting through life. Over my lifetime I have had persona's.

One such persona made friends with a girl at university years ago, her partner told me I was her best friend. My learned definition of a best friend is that they are someone you  tell important things to first before you tell anyone else such as getting married, a family member/pet passing away who was important to you (who maybe the other person knew too) etc.

A few years back t found out this person who I had seen as my best friend for years had announced on social media (tagging in those had told first) that her and her partner were getting married, my name wasn't included in those tags. I was hurt at the time but had other things to deal with. I was having to make the most heartbreaking decision to have my beloved soulmate and assistance dog PTS, so I just didn't go on social media and focused on making the most of the last weeks with my special girl (inculding having one last birthday for her).

When they eventually visited to tell me I couldn't care less as I was suffering from severe depression as in my mind I'd lost my best friend and soulmate who had never left my side for 15 years. In my mind (ie my dog) 'the (human) friend' had already made her declaration to everyone she knew that I was not even on her 'best friends' list by not tagging me (or even texting me in private first to let me know). Turns out they only wanted me to have her dogs so they didn't have to pay kennel fees while they got married.  I have an existing dog still and he (fortunately for me as i saw it) was acting off and growling at them suddenly (he'd know them from a pup) and my mobility wasn't good she seemed to see it was going to be a struggle and I would be unable to seperate them if the dogs started fighting so changed their minds about leaving their dogs here.

I wasn't expecting to go to the actual wedding (they were going abroad for it) it's the fact that her announcing it to everyone EXCEPT me in my mind gave me the very clear message she no longer considered me even one of her best friends. I can disconnect form people easily and once I do that's it I don't know how to forgive and 2 years later I want to end the friendship, as she still visits to bring birthday and xmas presents so I have to continue to buy them too (though they never tell me if cards/money arrives or thank me for them - kids are grown up and haven't visited me for years). 

Do i keep the relationship so at least have ONE friend to be seen as a 'normal person' (none of my neighbours know I have autism only family and this person) so local people leave me alone and I don't become a 'target' for being considered weird.  or end it because I've already mentally disconnected and dissociated. I've even created a new persona' so the persona she was friends with doesn't exist anymore anyway and she isn't a friend of the 'new persona'.  I have no interest in making new friends only be to let down further on in the future and have lost all interest in maintaining relationships due to this cos whats the point?' if you don't know someone after 20 + yrs when do you know them? (my brothers partner also cheated on him after over 20 years together and tore apart our family 3 xmas's ago too).

I see nothing wrong with letting her think I'm dead or died from not answering ever again and using possible future scenario's to make her think that (killed during a looting if Brexit went really bad) but NT's find this shocking and say I can't do that but seem unable to suggest an acceptable alternative.

so how would other autistics handle this  if they no longer wanted to be friends with someone as the other person had made it clear first that they no longer considered them their best friend (and was just seeing them as a charity case cos they had no other friends)?