Pretty much every worker in an office or like work environment has a to-do list. But how many of us actually start tickling the items off? Not many probably as the pressures of everyday work start piling up the moment you walk into the office. Basically, you are too busy getting the important stuff done and have no time. If this is the case, you have set unrealistic expectations for yourself, and you need to give yourself a break.
You may go into work full of optimism but as the reality of the day dawns you have to prioritise and as usual that to-do list gets sideswiped for something more important or pressing. You need to reframe your mindset. Instead of focusing on what you have not done, focus on what you have done. It may be worth taking a look at the to-do list and figuring out what really needs to be done. If there is anything that does need doing, you could do that first thing tomorrow morning before anything else pressing falls on your lap.
Think about your day, were you sitting with your feet up sipping champagne? No, then you were doing what needed to be done at the time when it needed doing. You were productive, and you have accomplished many things.
Be honest about your limitations. You are one person; you are not a miracle worker. Maybe the to-do list can be shared with other colleagues to give yourself a break or maybe some items can come off completely.