When I started commuting to work I made a commitment to myself that I would ride everyday I could, however as part of that I also allowed myself to uber / taxi / take my own car once per month on average for the days where that just wasn't possible. It was great, I initially thought I had underestimated how often I'd need to use my get out of jail free card, but after twelve months when I reassessed how I was going I realised I'd only needed to use it about six times in the year, so on average once every two months.
Everyone's circumstances are different in terms of commute length, nature of the commute, access to other transport means and lifestyle aspirations etc etc, but having such a commitment can be quite a motivating factor. If you allow yourself the occasional out, you won't feel so guilty about using it, and might find that you can more easily sustain your car free(-ish) status.