Tigger, I am not sure whether you mean obsessing as in a psychiatry-talk obsession or obsessive thinking, or a manic obsession with something/someone: more like a fetish.

Basically, do you like your obsession/obsessing (too much) or not? Are you afraid that mania might be around the corner if you don't can a handle on it?

What I personally do is just let it be and try to be kinda productive at the same time. So I might not be in control what I do, but a least I make the best of what I do. Some control should be possible though: I do everything to focus on something that is useful. It is really trying hard to focus with a camera on something particular and somebody else tries really hard to grab your camera. It can be done, to some extent.

If it involves spending too much or other risky behaviour I would (with low or moderate severity and no really health-threatening risky behaviour) do things like let someone be in charge of my money, so lending it to someone you trust or if you can get (IOU) guarantees and being in places where people can always force you to get help if things get more ugly.

Distracting your mind by stimulation from your surroundings might also/really work. But just observe, don't interact. Just be in as much as a safe place as you need to be (maybe no shops, maybe no traffic), but with lots of stimulation. Trains are pretty safe and I think it works absolute wonders. When I manically wander and wonder I always visit busy railway stations. The whole experience reminds me of "Waterloo Sunset" (The Kinks/Rat Davies).

Eventually it will calm you down. Sure thing.

Edit:
If your trust could really be "pathological", maybe wait a little longer before you really know your therapist. Likewise, if your paranoia is really strong, just be honest and frank.