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?

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.

For ur situation, do u understand this colleague well? If u know her well n knows that she is someone who does not take advantage of ppl.. it is probably just a once off situation where she went to the pantry and take some water n just nice bump into someone who just chat with her on smt.. n she can't denied..

In general.. there r all sort of ppl at work.. so u need to judge urself whether is this someone u want to help.. there r ppl who generally takr advantages of others n ppl who dont.. n will help u out when u r in need too.. so it is really for u to judge.. n if u feel uncomfortable in helping someone.. than I would say not to help.. n sometimes we also need to give n take alittle..