Being fulfilled and being complacent have crucial differences. The greatest difference is simple but must be stated. To me, being fulfilled means that you are getting what you want out of life, while being complacent means that you are wanting more, still missing things, but settling anyway.
For example, you might be working an office job that you do not actually want to do but it offers solid pay and most importantly, security. You understand that it wasn’t easy getting to this point, things could be worse, and pursuing what you actually want would be very challenging and risky. As long as you stay at that job while wanting to pursue something else, you will not be fulfilled, and quite frankly, you wouldn’t be happy either. You are settling and being complacent.
This is why it is important and beneficial to know who you are, know what you want, and consistently make decisions accordingly. You have to make a decision on either going for it or stopping wanting to pursue a different career. In this scenario, you either realize that the risk and hard work involved in pursuing what you actually want is worth it so you decide to go after it, or you realize that what the current job offers to your life overall is more than what you’ll probably get from taking that risk so you stop having that desire and decide to stay. For the first option, you may choose it because you understand that no matter how "good" things are and how much you have, what matters more is what you get out of your experiences and what you have. What is the point of having money if the things you can buy with it contribute minimally to your fulfillment? There are also many reasons for why you may choose the latter option, especially when you have more responsibilities and things/people you care about. If raising and supporting your child is the most important thing in your life and is what you really want, then you can very well decide that having that solid stable income is more worth it. If the lifestyle that the job enables you to have means more to you than the career that you actually want to pursue, then staying at the job seems to be more worth it. When you have thought it through, understood that you probably cannot have everything that you desire, and realized what matters to you the most, you stop falling into complacency; you start getting what you actually want out of life and start feeling fulfilled.
Fighting complacency and finding fulfillment is about looking at your options, what you get out of each of them, and what your deepest desires are. When you are fulfilled, you don't feel like that you are sacrificing, that you are missing out, or that something is missing from your life. You would look at the decisions you've made and the situation you've put yourself in and realize that whatever other things you didn't choose, even though you may enjoy them, like them, or have some lesser degree of desire for them, do not offer you what this thing does - they contribute less to your fulfillment.
Another thing to note is that this can be stated as understanding your priorities clearly, but it isn't stated that way because people generally feel like they are sacrificing things or giving up things when they think about priorities. While you are technically sacrificing and giving up things, you don't feel like you are when you think about it this way. This mindset is built in a way where you lay out your options and what you get out of them, then choose the one that offers you what you really want. This makes you understand clearly that you have lesser degrees of desires for whatever you're getting out of those other options and that you only have these options and this amount of control. When you think back on your other choices, you would think, "if I really wanted that and believed that I would get more out of that, then I would've chosen that," as opposed to, "I gave that up for this." The emphasis is always on what you want and getting what you want when you think about it this way. It is difficult to feel like you are sacrificing anything when it is super clear to you that it isn't what you really want and you are already getting what you really want.
Lastly, to emphasize, regardless of how you want to think about your life and your decisions, it'll make your life much easier if you understand and accept that it is often impossible to find an option that offers you everything that you have any level of desire for.