You may feel better if you know this kind of mistake is very common, it may happen to experienced traders too.
Some people call it risk spike.
I have made a similar mistake. Well, it is not exactly position size or no stop loss like your case. But a mistake I was aware of. I didn’t follow my plan. I didn’t do what I knew I should do, because of greed and excess of confidence. The result: a huge DD. My acc went from $62K to 27K. It hurts very, very, very bad!
I made this stupid error, despite I am very disciplined, and I am a long term meditator (having been meditating for many years).
After this very stupid and hurting error. I decide to read my trading plan every day. And the first paragraph of the plan says: “I follow this plan to the letter. If I want to change something, I must analyze it and make a decision in writing, then I will modify my plan. I can do something different only when the plan has been modified”.
I read my plan every day. Since I think it is the only way to make sure I will follow my plan.
Now return to your situation:
If I were you, I would use this technique: write with a pen on a paper this affirmation 15 times “I always use stop loss and riks only 1% per trade”.
Write it 15 times before you go to bed at night. And 15 times before you start trading every day.
Write it on a paper using a pen.
Do it for 6 months at least.
Wish you great success