I understand you well. My husband was much like your SO when we had 4 young kids at home...........He worked swing shifts and each week was a different shift. Day shifts he would come home, play with his toys, motorcycle, mow lawn, take care of yard and he did well with the kids but he would never go to any parent-teacher conferences, or social events that we were invited to where he did not know anyone. I went alone.
He picked up all the overtime he could get at work so there could be weeks when he worked 11PM - 7 AM 7 days in a row and I had to keep things quiet in the house with 4 kids etc. He was a light sleeper.
When he worked the 3-11 PM shift again it was the up keep on the house, snow shoveling, oil changes on the car etc. Then off to work he would go. So I often spent 7 evenings in a row with just the kids. I cannot say I was happy with this routine as I had the dream of a Hallmark Home..............everyone happy including Mommy.
I joined a volunteer group but again there were socials and my hubby would just not go.
I finally decided when our youngest was 6 years old I wanted to go to nursing school. So I did and my hubby sold one of his prize possessions, a classic car, to pay my way through school. He was very supportive of my goals and when I took a full time job in town he did his share at home.
For me I believe part of my problem was excepting that dreams sometimes have to be rearranged and I love my husband so with time I came to accept my life and my need to take care of me. I had a carreer and my family, life was good maybe far from my dream but it was and is good.
I know that lonely feeling you speak of.