I am so sorry to hear you lost your horses, that is so toughWell, sadly, this summer I had to say goodbye to my two oldsters.
Komet my former eventer was 31, with Cushing's, cataracts and arthritis. He finally got to the point where he couldn't get up and down in spite of pain medication, his rear suspensory ligaments could not support him any more. He just stood in one place in the pasture, and having been his person since he was a baby, I listened to what he was telling me - he was ready to go.
At the same vet appointment, Fiona (27 OTTB) who already had heart valve insufficiency and a benign thyroid tumor, was diagnosed with EOTRH, which caused gum pustules and was painful for her. Some horses with this can get their incisors removed and do well, but with Fiona's other health issues and her age, I didn't think it would be fair to her to put her through a major surgery. So after taking a couple weeks to decide (wanted to wait until I was less emotional after euthanizing Komet) I let Fiona go, too.
Both of their deaths were very calm and peaceful. One second eating treats from my hand, the next second gone.

I think with time, obviously, it does get easier. And I am glad to hear you have already found a new horse to ride! She sounds lovely. I agree with you that it is best to try to rely on our knowledge of our horses and make that judgement call, I know my boss had to go through that with her mare of 21 years who got laminitis really bad. Ariel always fought it so she kept fighting for her, but finally my boss felt like it was the right time to call it, before her mare began to suffer over the summer heat once again that was coming up. This was a few years ago now, but still. It's hard but necessary to ensure we give them the best quality of life possible.