The one bit of advice I wish I'd received much earlier in my career is:
Saying yes to one thing means that you are always saying no to something else!
It seems pretty obvious when you think about it, as time is finite. So every little task you take on, is consuming a part of that finite resource. When you're starting out, you're probably 'time rich / cash poor' which is probably the worst combination as it almost always rewards taking too much on. You've got more time and you're still learning, so over committing a little won't hurt surely? The worst possible thing then probably happens, you're rewarded for your efforts with promotion and salary increases.....and the beginnings of a bad habit is quickly formed / re-enforced.
In the beginning you are probably saying no to your leisure time:- I really want to get this done and I'd only be sitting at home and it's only this once / small thing. Slowly, as your career develops, you'll most likely end up with more things than can be done in the hours available (often all the hours in the day, not just work hours). If you're in this situation and that urgent, important work comes along (it always does), then of course you'll need to / should do it. But the next question, after saying 'yes', is 'What other task can be delayed to make space for this urgent work?' Sometimes just asking this question might highlight that that 'important' task isn't quite as urgent as it first seemed in relation to everything else you're currently committed to. Either way, the outcome should be you feeling confident you're still able to deliver on your commitments without sacrificing your social life / health.