The more reactive or service-based your job is, the more you are likely to get stuck working overtime, because problems can land in your lap at the last minute and need urgent attention. If you aren't in that kind of job, then the problem is volume of work and poor organisation - either your own or the people who give you the work to do.
What do your colleagues do, especially
the working mums as typically they are the ones who leave on time? If they all manage to leave on time and arent sobbing with stress every day or on disciplinaries for poor performance, then it is you that is the "problem". Maybe ask a few colleagues who leave on time how they do it (Although in a lot of cases the answer is they go home and switch on laptops or attend to emails at weekends.)
I think many jobs have an expectation of some overtime occasionally but if it is significant and it is every single day then there is a staffing or a management problem.
If you've always worked overtime and you do it every day, i think it becomes a habit and whatever work you've got somehow stretches to fill the time. If you are obliged leave at 5.30pm, then unless there's an emergency you can usually manage it by being hyper-productive during the day.
You can wind the situation back, but you have to retrain the people around you to know that overtime is not your norm, otherwise someone will always be bothering you with a last minute request. One idea is to book a gym class after work, maybe twice a week, and make this widely known so that you leave bang on time those days. Without fail.
You have to push back on whoever keeps you late. For example, don't take on ANYTHING new in the last half hour of the day. Reply to emails and verbal requests n this 30 minute window with, "I'm just wrapping up my work for today. I'll have a look at your request by 9.15am tomorrow and let you know when I can get that work done." Then wait and see if they explode at you. Over time, people will realise they can't dump stuff on you last minute just because their own organisational powers are rubbish.
If you manage your own time during the day, you also need to keep a clear To-Do list, with all your activities on it, and an up to date calendar. If you have regular tasks, one idea is to mark a regular slot in your calendar with a realistic time frame to get the work done. In that space of time make it your sole focus. If that is impossible because your job requires you to keep an eye out for urgent messages it can be insanely disruptive. If you have IM at work eg Skype, if your calendar has a meeting in it then it should switch your status to "Busy". As back up have a phrase on your paste-board like, "hi, I'm busy with X until X o'clock, unless you need me urgently can you send an email instead of IM please?". If you are being constantly disrupted by customer requests and your priorities are chopping and changing all the time, then you need to talk to your manager about how to handle that. I had a colleague once on my team who replied to customer requests almost instantly, kicking off huge email chains which I needed to weigh in on to stop stupid things being agreed to. It was so time-consuming and my inbox was always groaning with unread mail. Email overload is a HUGE problem in some jobs. Monitor how long you spend on email and ask your manager for help if it is eating up an unreasonable portion of your day.
Lastly and I say this with kindness as i am AWFUL myself, but ask yourself if you:
- are a perfectionist. This can mean finishing tasks or wording emails can take longer than for a non-perfectionist.
- prevaricate and put off difficult or big tasks because you like the comfort of clearing off the easy wins first
- chat too much to your colleagues
- feeling entitled to be on your phone checking Insta or watching your puppy on the Doggie Cam or whatever for a portion of the day. A minute spent not working, is a minute you owe back to your employer.