After months of fighting to save their 11-year marriage, Peter 38, filed for divorce in March, citing ''irreconcilable differences'' as the reason behind the split.
But despite knowing it was coming, Jennie, 39 - who met the 'Twilight' star on the set of TV movie 'An Unfinished Affair' in 1996 - says the end still took her by surprise.
She explained: ''[It] was something we were working through together. As anybody who's been through it, it's always kind of shocking when it ends up finally happening. It was sad for our family unit.''
The former 'Beverly Hills 90210' star is currently promoting her new reality show 'Jennie Garth: A Little Bit Country' - which centres around life on her farm with daughters Luca, 14, Lola, nine, and Fiona, five - and has found it impossible to avoid questions about the divorce.
She told US show 'Today': ''That is probably the hardest part about [the split], actually, the timing of everything.
''When I started to take this show on, I wasn't headed down this road. It happened in the middle of filming the show, and now part of doing any kind of job as an actress is following through and doing the press circuit for the show, doing interviews, and people want to talk about it.
''And, I am okay talking about it, but it's certainly not ... something you want to talk about.''
But she denies the show had any part in the split, as well as reports she was jealous of Peter's 'Twilight' success.
She insisted: ''There was nothing like that going on. I was very happy for him and so happy to be at home with the kids and enjoying my ''early retirement''. I didn't want to work at that time. I just wanted to be at home on the farm.''