I'm dating two people, but one of them has dependency & insecurity issues that give me pause

I recently entered into a polyamorous relationship with two people, T and S, who have consistently made it clear that their relationship is an open one. Upon starting our relationship, I quickly became aware that S has mounting insecurities and fears, and is completely unable to be by herself. She has a low sense of self-worth and individuality. T and I are unable to spend time together without running the risk of her having a nervous breakdown or a panic attack. She can quickly revert to a child-like state, assuming malice on T and I's part, assuming that we are trying to oust her. This is despite her reassurances that she is naturally poly and not monogamous. Despite the fact that I have put in a lot of effort to form my own relationship with her. And despite the fact that none of T and I's dates are without her knowing. There is no conspiracy against her, and we have been as transparent as possible when communicating with her. She has become so naturally dependent on T that I doubt her ability to function as an adult if they were to break up. So my questions are: Is it possible to be in a healthy polyamorous relationship with someone who is that uncomfortable being alone? Is it possible to be in a healthy monogamous relationship with someone who is that uncomfortable being alone?

I am not able to say in the general case whether it is ever possible for anyone to be in that kind of relationship in a healthy way. But that's not really what you need to know. You need to know whether it's healthy for you to be in this relationship.

If S's behavior is preventing you from having a healthy relationship with her and/or with T, then it's not a healthy situation for you. Whether such a situation could potentially be healthy at some point for some person is irrelevant. Don't get stuck around trying to wait out or resolve something that's not working for you because some blogger said that it's possible for it to work.

Talk with T about the way catering to S's emotional demands is impacting you, and what your needs and boundaries are. If your needs can't be met or your boundaries can't be respected due to S, then it's not a healthy situation for you.

Sure, maybe some other hypothetical person would feel fulfilled by helping S feel secure, or is emotionally wired such that S's dependence doesn't bother them. The world is a rich tapestry and it's not for me to say that S is someone with whom no one could ever have a healthy relationship. But you have information about what is healthy for you, so act on that rather than my hypotheticals.