I feel like everyone takes advantage of me at work because I'm nice. Should I change my behaviour to stop people from giving me undesirable tasks?

Set your boundaries

Decide on your to-do list, be very clear on the three things you want to accomplish every day (of course, when it's fighting fire season, we will be lucky if we can accomplish even one task), set boundaries, help your colleagues if you can, decline and SAY NO if you can't

You are not a mean person if you say nope. We need to help ourselves first/make sure our tank is full before we can extend a helping hand to others. So never feel guilty about saying nope

Nope you should change job.

People take advantage of you because they are that type of people rather than because you are nice.
Other nice people won't take advantage of you but instead boost you up using your nice character.

Changing your behavior might become a 1 way road.

You could politely mention that you’re happy to help out after you’re done with your current work as you’re quite loaded with work your end as well. This way, you maintain your helpfulness while ensuring the priorities will remain your own work. Kindness is a virtue and you should not let this change. Just need to say no sometimes and be kind towards yourself as well.