Hi there,
Not sure if this would work for you, but my therapist and I would set aside some time during the day to meditate on those bothersome thoughts I obsessed about, and then after we were done I made a deal not to obsess anymore about them. It's very hard to do at first, to set a chunk of time aside, but it works!
You don't even need a therapist to do it, you can do it by yourself as well. Just start with whatever amount of time you need, say, 15 minutes...then you'll find that eventually you'll get bored by your obsessive thoughts, and won't want to think about them anymore!
Other things that help me are distractions, such as listening to music or hanging out with friends.
Wishing you the best!