Look at these, please
1 I thought I might go
When it comes to 1 we use "future in the past". I means that it was possible that I would go later
2 I thought I could go later
Does 2 express at the same time ability and possibilty or could is only used for ability and might for possibility?
English has a number of possibility words, you have two of them but there are more (may, and even 'should' in some dialects) and they are all almost-synonyms. It's not that one only expresses possibility x and the other expresses possibility Y. What they mean exactly varies a lot, from speaker to speaker, and with significant overlap. What is broadly true is that they are a spectrum from "almost certainly no" to "almost certainly yes" and with every word indication what part of the possibility spectrum it is on.
What the speaker means with their words you'll have to gauge from the context and their way of speaking. A cautious person used to a lot of sarcasm in their local dialect might say "this might happen" and mean "this is certain to happen"
Cambridge has a good summary: https://dictionary.cambridge.org/grammar/british-grammar/could-may-and-might
