I will take slightly different approach.
A healthy, supportive, loving relationship is by nature a healing space. So my take and based on my experience, my marriage was the biggest, most influential in supporting, promoting and creating a safe and healing space for me. I got married later in life too so that helped me as well to see this difference.
Where I think most people may struggle is what does this mean for me? Like if god forbid my husband and I split today, am I back to square one, struggling, extremely symptomatic and unstable/reactive beyond help? Of course the answer varies depending on the relationships involved. In my own life, the strength I gained, the growth I have done, the risks I took (including therapy), all give me a real chance of grieving deeply for the loss of my marriage but it gives me much more strength to move forward. In short, I do not see going back to not knowing, experiencing what I have now, remembering of what I know today. Where trauma broke me because I was a child, the experience of my marriage helped me understand me in relation to another person - who BTW is strong man who knows his boundaries and has his own growth in the process of us being together. The creation of the relationship was significant contribution to my healing but without my own intelligence and willingness to risk it all, it would not have happened. I could just as easily stay and close my eyes and just live in a bliss of ignorance. But I (by pure luck) chose a man who was not like my mother. This is a huge for me. However, he is not perfect trust me. He has his own issues who does not? But my compassion is large and can accommodate my own recovery and others too.
IMHO, a great marriage is much better allowing, paving and nurturing growth and healing and a good therapist is much faster way of reaching the same thing. The main difference is marriage is not focused on my growth and striving for differentiation, integration and self actualization, it is a group (spouses) effort and commitment and those things are by products of great marriage.
A good therapy, on the other hand, is very much all about me and how I can reach same thing much faster cause it is focused on me and I will use every fabric of my being to learn and focus on differentiating, integrating and maybe possibly actualizing but not really...no one reaches that level and if they do GOOD FOR THEM. I will add a caveat from my own experience, a great marriage makes therapy easier burden on those heavy, crazy, unbelievable experiences and extreme disintegration and dissociation process of working on childhood trauma. One must breakdown and see the cracks to heal and no better than to have a person who loves you and support you even when you are that broken. I would not underestimate the power of love in healing.
At the end of the day, some result of great marriage and good therapy is that you realize you are alone ultimately and will always carry your own burden and still you will choose to be with others and make the best you can everyday in this struggle.
A huge red flags are when one just transfers issues from the past to the present spouse whether those are unmet needs of attention, admiration, love, resources, to offset abandonment or emptiness or use others as coping mechanism or missing attachment etc etc and other earthly things that were thwarted in our early days and re-create morbid dependency or co-dependency like addiction...then one is not in a healthy relationship but a re-play of the old tune that hurt them.
