i think that if you do something physical everyday, then you don't have to be so hard on yourself when you do go to the gym and don't do a super long, hardcore workout. in talking with different personal trainers, a couple of interesting things have come up.
1. cumulative exercise-this is helpful, especially if you are busy-it means that all the exercise you do in a week ADDS UP! But it doesn't just apply to going to the gym. this can be taking a walk on your lunch break, walking up the stairs instead of taking an elevator, biking to work...etc...
2. our bodies need different types of movement! not only does doing the same routine at the gym get boring, but it turns out to not be the best idea. the trainer who told me this is a sky coach and he said a lot of top athletes incur injures because they focus on only one thing. so, mix it up! even if it's not at the gym, it does count!
the main thing is to move your body.
...that was longer then i thought it would be, sorry!

Plan, plan, plan. For me, at least, plans get rid of guilt. My exercise routine is always the same, and I contemplate tweaks for days or even weeks before I implement them. I never let myself do less OR MORE than I planned, because I know that once I open the door to the option to do more, I will start to feel like I "should" every day. So if I finish a workout feeling like I am not as exhausted as usual, I note it and think "I maybe need to increase intensity"--and if I feel like that more days than not, then I do. But I change the plan, I never spontaneously challenge myself.

I try really hard not to think like that because inevitably there will be the day that I only have time for X. And if I think that if X isnt enough then I might not go at all.

Yes, I'm this kind of personality too. My best is never good enough. I have never been able to determine my "best effort". Did I do my best? Really? I couldn't have done just a little more?

I combat this by setting a goal for myself each and every time I set foot in the gym. Sometimes that goal is just to get through a session, like if I'm sick or having and off day, and sometimes that goal is to surpass my previous mileage or time. In spin class my goal lately has been to keep up with the guy beside me! It's great motivation. If I achieve whatever goal I have set, I AM successful and even I can't argue with myself about that.

It's never about what the others are doing. It's only about what you have done in the past, and what you can achieve now.