I went through a stage where I had exactly the same problem as you. I'd cancel things or avoid social occasions because of OCD thoughts. I understood that it made little to no sense, and that my fears were totally irrationnal, but still the OCD made me so afraid I preferred not to "take a chance".
But you have to realize: the thing is there's no risk of anything like that happening. You're never "taking a chance"; you don't have to fear that you will lose control over yourself if you let your guard down against OCD fears. That's all in your head and it doesn't mean anything.
You say that things get better when there's no planning period, when it's just an immediate decision? Maybe you could try something like each day, you plan to do something one or two days later. Then, after a while, you plan to do something three days or a week later. And then, when you can get comfortable enough with this, prolong the time a little bit again. Each time you can plan something nice, enjoyable, something you like doing a little further ahead. After a while, it might get easier to ignore the anxiety and the "what if this happens" scenarios.
By coping with that and trying to ignore the "what if this or that happens" scenarios, you are not giving in to the OCD. You are actually fighting it, and trust me, the more you fight it and resist it, the better you will get at dismissing the OCD thoughts as just OCD thoughts --irrationnal, meaningless, and NOT what YOU want. And remember: you are not alone: there are tons of people out there struggling with OCD and, fortunately, seeking help. If you think you need it, you could see a psychiatrist, or maybe get a book. Whatever you feel suits you. Anyway, hope I helped. Good luck!