Everyone has personal limits.
I have one. I am not willing to parent children and be in a polyship at the same time. I just do not want to. No, I do not want to "learn how to polyship with children." I would rather raise my kid and be content in the Closed Polyship of 2 with DH until kid is grown. Then we can talk about our next phase of life and how we want to live it.
To me it sounds like you have a personal limit in conflict with a want.
- If you are going to polyship again, you WANT to be in a polyship that is open, and out. Because experience taught you that it felt yucky not to be.
- Experience also taught you that your time is limited. Time management matters.
(You wrote it in one sentence, I broke it into two. Because it is two separate things.)
You have stumbled on some personal limits. You could think about ways to address them.
You feel you are not brave enough yet to live Poly "out" to people. What can you do to get braver? Who can you share this news safely with? Do you have any poly friendly community around you?
Whatever you have pressing on your time right now (work? other rships? Commitments) leaves you X time available to devote to the tending of another person. This was not enough for your ex. How much time IS it? What kind of relationship can you have with X time available? Can anything else in your life be changed to increase your Time Budget?