Husband filed... feel lost

Hi- this is my first post here. My husband filed for divorce after 21 years.
 
There is no other outside people, I have never been with anyone else physically and as far as I know, neither has he.
We lost a child in 2003 and have had many deaths since. He seems to internalize these deaths and it's made him a different person... quiet, no sense of humor and he does nothing but sit in front of the TV on the internet. 
Four years ago I had an "emotional affair". I was NOT attracted to this person in any way, it was just a silly online thing and phone thing. We were childhood friends and he would tell me I was smart, funny and all the things we love to hear.
My marriage had been dull for a long time. My husband paid me almost NO attention and never showed appreciation for all the things I did. I am nurturer and I took complete care of my husband, my child and his kids also. His ex wife passed away, so I took 100% care of his kids. I did it happily with love. I worked full time, kept a clean house and raised all 3 (sometimes 4) kids. 
I had always missed where I grew up, and when I reconnected with all my old friends back home, I decided to leave my husband and go back home. Things had come to a head, my husband was depressed and I wanted my freedom. This male friend I had the emotional affair with there also- but I ONLY was a friend. he did NOT hit on me or try anything with me and vise-versa.
I am VERY good at boundaries with other men and Im only friends with males that are gay or that Im NOT attracted to. 
I packed up my stuff and left..... I was torn- I was happy to be back "home" and with my friends support (4 girlfriends and even a family member rallied around me for support) - but I missed my husband horribly. 
He asked me to come back, so i did after 3 months. It was painful and horrible being alone.... and I missed him and home.
BEFORE our separation, he was dull and not paying me any attention- but were were somewhat happy. 
SINCE I CAME HOME: My husbands unbearable... he's STILL upset about this emotional affair. Again: I NEVER cared for this other guy or even touched him. This may sound cold- but I just liked hearing nice things about myself... nothing more.
My husband has become obsessed with checking my FB, every email, every single online message. Although I have done nothing since Ive been back (and had done NOTHING before that).. he wont let it go.
Worse- he's gotten controlling. It's escalated to CONSTANT internet monitoring, and sometimes yelling at me.
he holds resentment over what happened 4 years ago- he wont let it go. He cant move on.
I have caught him online looking at adult websites- I have forgiven him and moved on... it;s almost like my husband is INCAPABLE of forgiveness. 
I have been staying because I was scare of being alone. Now for the 4th time in so many years, he's filed for divorce AGAIN. This time it's because of me being on twitter. (not doing anything- just tweeting)
I have the feeling the root of this is me being on social media. 
I KNOW he's using the divorce as a tool to control me. This time- I am over it and want to move on.
I am SCARED TO DEATH of being alone... I know I must do this... How do I get through adjusting my life at 45 to being alone? How do I stay away from someone I've loved for 25 years of my life?? How am I EVER going to let go??
If anyone has any advice- let me know.
 
-BethinMiami