How do you deal with regret?

Backstory: When I was in college I worked p/t where my dad did, for a small industrial electronics manufacturer. The VP there was chill and ate lunch in the break room with everyone else. He mentored me on investing and I started my first account at age 19, with $1500. Over the course of about 2.5 years, he gave me advice, tips, and told me equities he was investing in. In the summer of 2002 he suggested I buy Corning Glass as they were absolutely beat up, undervalued, and would bounce back in the future, he was sure of it. He had nearly 50K invested in them, so I bought 500 shares around $1.95 and added another 300 shortly thereafter, bringing me to 800 shares at an average of around $2.95 each. I sold some after college to pay off debt, but still held around 500 shares until I was strapped for cash after my divorce, and sold the remaining, keeping only 2 shares and applying DRIP, so now I have like three. I say all this because I am having deep remorse and regret, and it's causing me severe stress and anguish, knowing that I could be debt-free with no mortgage if I just held those 800 shares. I know that's a lot of what ifs, but how do others deal with feelings like this? Thank you for listening