Amy, this was a beautiful story. Thank you for sharing. I'm 39 and hope to have a family someday; I haven't found the right partner yet, and the longer I wait to find him, the more I worry about these kinds of things you express. But I'm sure that your kids wouldn't trade you for any other mom, and you wouldn't be the mom you are if you'd had them any younger.
About the Alps: it might be comforting to consider that perhaps a fixable health issue caused your struggles. I'm fairly fit and walk EVERYWHERE because I never owned a car, and yet even I struggled at age 19 in the Andes. Then, last year, I went to Switzerland and struggled with moving around in the mountains once again. Turns out, I have a severe iron deficiency, undiagnosed for ages because I wasn't anemic. Given that you used to exercise intensively, have had birthed (and maybe also nursed) 3 kids, and have gone through your entire reproductive life (with allllllll the bleeding that entails), your body might have very low iron stores. Maybe tell a doctor about Switzerland and ask them to test whether low ferritin might explain the exhaustion and shortness of breath. (That's my unprofessional tip, but iron deficiency has been a huge quality of life issue for me, so I hope it's okay that I share this!)
You sound like you do a lot to keep yourself healthy for your kids. I'm sure that this keeps your brain healthy too! Wishing you a very long, happy, healthy, vibrant life to continue enjoying all the love you've created. 💜