"It was so dumb," he said.
"Just a bone-headed move. I was young and genuinely thought it would help, but there were times I'd just finish a scene and go sob in my trailer."
Donovan says despite how hard filming with his ex was, the rest of the cast were "really supportive".
"Courteney Cox, Lisa Kudrow, Matthew Perry, David Schwimmer and Matt Le Blanc, were so compassionate and understood what was going on."
"At the time, I was like 'oh God, why did I do this to myself? But now I look back on it and think it was a great thing I did those episodes, on this great show, even during that tricky time."
