Investing yourself at engagement or marriage is too late.....you need to be fully invested to get to that point.
My question is also about the red flags. In a hypothetical situation, say your boyfriend called you the C word during a fight. What thought process would happen to you to make you ignore the flag? What if he's 25 and has never worked a day in his life. Again, how do you justify staying with him?
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.