Welcome Assika126,
I have just started therapy and am coming to realize that I have in the past used his crises and my habit of caretaking him as a way to avoid working through my own growth. I have therefore resolved to try to stop worrying about other people and start working on myself first. I am very grateful that I am finally am coming to have faith that there is a road to a better way of living.

You are working on the only bit you truly control!
I particularly want to know what a healthy response would be when my husband starts beating himself up in conversations with me. I find it painful to hear him say such negative, distorted, and untrue things about himself, especially when they arise from opinions he thinks i have of him - which I do NOT. I don't want him to feel like he has to bottle his thoughts and feelings up rather than share them with me, but any response I can make doesn't seem to help him or me, and I still feel awful and somehow wrong. When he acts helpless in his own fate ("I am too old to succeed and I am just a fraud. Nothing is ever fun or exciting - that's just gone from my life forever. I should just accept that and give up my dreams and do something I hate which is practical.", it's not right, it makes me feel gross to even hear it and I just freak out, I struggle to keep my cool and be present.
What do you do? What do you say to be supportive and still manage to avoid that sick feeling of getting drawn in? Honestly there have been times when I would turn to him and say, just talk to your therapist about it, I don't want to hear it tonight! The worst is when he starts in when I have somewhere I have to be, and I know if I break in with a reminder he acts like I don't care about him enough!
Remember you wanted to work on yourself . That is key! You can't control what he is feeling - only he can.
Honestly there have been times when I would turn to him and say, just talk to your therapist about it, I don't want to hear it tonight!
Sensible attitude, almost a boundary. Read up on boundaries so you are able to go about this goal without becoming controlling if he does not listen. What you said is fair but gets close to become controlling when repeated. The more clean and consistent you implement boundaries the less conflict you will have (Getting them into place is often not without conflict).
What can you do constructively? Not owning his problems is constructive so you are on a good track!
In addition you can help him dealing with his negative distorted thinking by NOT telling him that it is not true. Of course it is not true but telling him would be invalidating and likely drive agitation. A better strategy is to listen carefully, figure out what is the prevalent emotion behind his self rage at the moment and then to validate this emotion. This won't make him feel great but it will lessen his very negative emotion into something more manageable. He then will have less a need to express with exaggerated facts imagined shortcomings. You'll find a good workshop to get started here:
https://bpdfamily.com/message_board/index.php?topic=206132.0By the way, I have brought up BPD with him, not because I think he needs a diagnosis but because it might help to understand how we interact, but he thinks BPD is untreatable and doesn't feel ok about exploring the ideas. However, at my insistence, he has been in therapy for several years with someone he mostly trusts. How honest he is with his therapist, I don't know. Sometimes I don't think he even know himself who he is or how he is feeling.
BPD is well treatable but it takes a while. It is good that he has build a relationship to a T. At this point it may be best for you to focus on your part. Learning and exercising validation and boundaries (they are skills and take practice, practice and practice) will help you to handle the situation better and plan the next steps.
I can't say the least thing negative to him or it will send him into a shame spiral which I will have to coax him out of! Anyway, thanks for listening and for being here.
Once you got a handle on validation communication will become a lot easier and you will be able to communicate negative facts.

a0