I've got OCD, and it definitely has an impact on my hobbies. Every one of my own cars I've restored has fallen into 1 of 2 camps - 1: planned to be a runner then got carried away with details and a need for perfection that it has been restored and/or upgraded to a point where it is just about as perfect as I can get it and it's turned into a shelf queen or 2: I've failed in some way and I can't even look at it anymore without being disgusted with myself. Perfect examples of both: My Monster Racer started out as a light resto to turn into a nice runner, ended up replacing just about every part with a new vintage part and restoring what I couldn't replace to within an inch of its' life and then getting every period upgrade I could; and my Zahhak/TRF201 where I've built it up with every upgrade I wanted, took a huge amount of time and effort to build it to the best of my ability, took ages trying to decide on a colour scheme for the body, made a mistake, stripped it, started again, got some bleeds that I didn't notice until I'd gone over it with the backing colour, now I don't even want to look at it and have lost all interest in it.
Doing builds/paint jobs for other people is probably easier in some ways as I can't give myself that opportunity to turn my back on it - I can't let someone else down, it's not an option, so I have to keep going with it. This has led to me even replacing bodies to start again with at my own cost to ensure I could deliver something suitable, and in cases where that wasn't possible, telling them I don't want paying as it's not good enough.
I totally relate to @Badcrumble where there are things I've not started because of a fear of failure - there are many projects sat in boxes in my garage that are in this state.
In the 1:1 world I'd say I have things slightly better under control, which I'd have to say is down to a mix of medication, marriage and psychological help. My OCD gets exponentially worse according to my stress and anxiety levels, so the meds help keep them manageable, but being able to realise when I'm starting to get obsessive and try to make some changes to reduce my stress levels is a big help. For example, I used to be one of those people who had to have every jar, tin, bottle, packet perfectly aligned in their kitchen cupboards and it would totally stress me out if they weren't. Now I do the unpacking every week and I'm ok with things just being lined up and tidy. If I notice I'm getting obsessive about it I'll make sure I take some time later that day to do something I enjoy to lower my stress levels. Being married to someone who is inherently untidy has also helped, as after 20 years together you realise you just can't spend 24/7 tidying up after someone! So on the whole, I have learned to accept that tidy is ok and I have to tell myself sometimes "just walk away from the cupboard, it's not going to kill you that it's not as tidy as it should be."
One positive of it all though is that when a friend or family member moves house or needs help I get to go nuts! A while back my brother in law had to go to hospital for a bit so me and my wife went and tidied and cleaned his house for when he got back. This meant I got to get the carpet cleaner out, the bleach, degreaser, polish and completely straighten out his kitchen and bathroom cupboards. Another example of this is when we go to a friends for dinner or a party. I'm really not great around crowds of people as I also have social anxiety as I'm an introvert, so I'll happily spend my time in the kitchen washing up, which will usually result in the home owners saying goodbye to everyone and coming into the kitchen to find everything has been washed up, put away, cupboards sorted, glasses polished - we don't always get asked back, but most of the time we do, and often I'm just pointed in the direction of the kitchen and left to my own devices!