Most people feel bad at staying in touch, including the people you've drifted from. Swap "I" for "most people" and reach out anyway.
Most of my conversations with people who want to get better at relationships start with a confession. "I haven't done a good job staying in touch with past clients." "I'm never sure what to say." "I feel bad because it's been so long." My response: good news and bad news. You're not a unique snowflake. Replace "I" with "most people." That means you're not alone, the solutions already exist, and most importantly, the people you've drifted from probably struggle the same way. Losing touch is almost never one-sided. The project ended, you both moved on, and now you're both sitting on the same guilt, each assuming the other is annoyed. Two people, both waiting for permission. Might as well be you who breaks the ice.
Years ago I spent ten minutes unloading a decade of anxieties on a coach, convinced something was truly wrong with me. He chuckled and said, "You're so typical." Then he said it four more times. At first it stung. Then I calmed down. He saw high-performing founders like me all the time, and there was nothing uniquely wrong with me. I now pass that gift on three or four times a week.
"Hey, you popped into my head today. How's everything going on your end?"