I mean first you go to cercei and directly tell her that I have found your secret. The secret which will destroy her and her children lives for ever. Okay we get it, you are an honourable man and want to save her from robert’s wrath and you told her so she can run away. But first atleast take your family back home to winterfell. Then make some arrangements for your own security and then tell her.

Then after the death of robert when renly asked you to join him, you should have joined him. Stannis was away at that time and renly was also a brother of robert. Atleast renly was much better than joffery and also renly had a better claim to the iron throne than joffery.