I was constantly moved on new projects, with new technologies stack, ranging from PHP, Python, AngularJS, Struts, Spring Boot, Vue, SQL Server, Node, React. You name it. ... I never received any guidance, training, or support. ... Is this just a sign that I'm not cut out for the job, or that I'm perhaps too old to keep up (I'm 42)?

Not remotely, no. I would say that it is a sign that your management (the people who move you to new technologies without guidance, training, or support) is not cut out for the job. They are exploiting you - as you probably know. If you were new to the game, in your late teens or early twenties or after a career change then maybe you might consider that an acceptable trade-off while you garner enough experience. As a more experienced programmer it isn't clear why you would continue to put up with this. In your shoes I would be actively looking around for a position with a better employer (or to strike out on my own but given what you've said about your current psychological state that's probably not a great idea just now).

In summary, it's not you and it's not programming; it's your employer.