Hy
I'm 35yrs old and my lover is few years younger than I am.

I'll tell you everything from beginning on how we met so you might understand better.
We've maybe been together for only 2 yrs but we've known each other for over 6yrs...it'll be 7yrs soon. This is not some love that just happened over night.
We've known each other for long time and over yrs we became very good friends. We didn't got together till now because when we met he was already with someone else. I've always loved his kind nature and personality,he would always treat me and everyone else with love and kindness. We even had so much in common,even back then when we were friends...same goals,same passions. I've finally thought that I've found myself a keen spirit and a good friend. Over several years,my feelings for him grew but he was still together with that woman, so all I could do is wish him all the best in love and life. He never knew that my feelings changed back then because he was with someone else and happy,I had no rights to interfere with that.
He told me he's gonna go far on a trip there to see her again and everything seemed fine.
But a week has passed,and another..and there was no answer from him. I started to get really worried :/ Then I found out that that woman broke his heart and soul & he literally ended up in hospital with a heart seizure. :'(
It broke my heart completely hearing that someone I cared for dearly ended up with similar experience as I did even before I met him. :'(
We continued to be friends and kept being there for each other.
On the year we got together(I was around Xmas), at the beginning of that year I was going through some extremely hard times because I had loss in the family :'(
I was going though hell for months
He was there for me through those awful times and over time,our love grew even more,till one night he told me he loved me. Our love isn't just someone that came over night....we knew each other and cared for each other long before that. I knew that I've found my soulmate because we would fulfill each others hearts and lives. Our connection was so strong,not just emotional but also spiritual.

But then...this happened :'(
Around 2-3 weeks before it happened,I was having constant messed up nightmares that made no sense...my own inner feeling was trying to warn me that something is wrong...I only wish I could've interpreted it on time
While he was in hospital I found out that he also lost last 3 months of his memory...under which there were some vety dear memories we both shared :'(
He never regained them.
When he got out of the hospital, he asked for a break,to get on his feet,to recover properly...and I gave him all that because I know that such things need time
After those 5 months passed,he came back to himself again...it was if the man I knew and loved never left. :"❤

And now due to that stress he's been going through lately,he had switched back.
I'm no doctor nor haven't talked with his psychiatrist but to me it looks like a mix of illnesses...at least from what he told me and from what I've experienced.
It looks to me that every time he deals with extreme stress his personality switches,and in complete personality disorder way as Pegasus mentioned...but this personality is the temporary one because the real him is the one that's always active when everything is normal....I know,it sounds like a complicated mess it's not normal BD, I never said it was.