I had a very similar experience, discovering my ADD in seeking help for my son. I was 40.
I recommend you get a diagnosis for one reason – YOU!
You said that it would not make any difference to know for sure, but from what you have said further about being alone and depressed, your diagnosis is your first step to shedding all the negative self-talk you have learned over the years that is at least in part interfering with some foundational areas in your life. It is admirable that you have taught yourself to cope, however your trouble with relationships indicates that you are still struggling and have not yet found the keystones for the life you dream of. You might also find it helpful to go on medication for a while as I did. I was able to relearn better skills and self-confidence, and retained that learning after going off the meds. I now manage when necessary with different herbs – lemongrass is my main go-to, but you may need to try a few different ones until you find the one(s) that are compatible with your unique chemistry.
One thing I want to say to you from the heart – as you rediscover and create yourself as the brilliant, different-thinking person you are, it will sometimes be hard and require a little mourning for the past you who struggled when it seemed everyone else had it all figured out (they really didn’t by the way). The fun begins when you start to realize ADD is a gift that makes you strong, creative, adventurous – whatever you desire. Oh and incidently, lucky you! It took me years to stumble onto ADDitude!