The red flags you already saw
Almost everyone who ends up hurt can name something they noticed early and talked themselves out of. Today you find yours, not to feel stupid, but to learn how your mind explains things away.
You did notice
Looking back, most people say "the signs were there". Usually they were, and usually you saw them. What happened next is the interesting part: you explained them.
"They've been hurt before." "They're just stressed at work." "I'm being too sensitive." Hope is very good at finding explanations, and when you like someone, your mind quietly looks for evidence that you are right to.
This is not stupidity. It is how every human mind works. The skill you are building is not noticing more. It is believing what you notice sooner.
Psychologists call this confirmation bias: once we want something to be true, we give more weight to evidence that supports it and explain away evidence against it. It is one of the most reliable findings in the study of judgement, and nobody is immune to it.
Pick one past relationship. What did you notice in the first month that you talked yourself out of?
How did you explain it away? Pick any you have used.
What would you say now to the version of you who noticed it and let it go?
Be kind. You were doing your best with what you knew then.
Take this with you
The skill is not seeing more. It is believing what you see, sooner.