They keep asking you to do more work because you keep getting it done.
YOU COMPLETING THE WORK is a signal that you have capacity to do it, and thus they might be able to get away with slightly more.
Effectively, you need to FAIL to get them to stop. Establish a boundary where if you are assigned work, it's unable to be completed after that threshold without other things being impacted.
Let them know when they give it to you that you will try.... and then just let it fail the deadline. Let it be late.
If they question you, just say you tried and it took longer than expected.
Once this happens, they will understand your capacity and stop increasing it.
A managers job is quite literally to manipulate you, a human, much like a software developer manipulates lines of code.
They will almost always try to keep piling things on to over-achievers, knowing that they'll keep taking it without confrontation.
If you think about it, if you were in their shoes.... Wouldn't you want to see if you could pressure an employee into 10-20% more work for free? You're limited on headcount / budget, so you know you can't get another person... maybe have Steve do it. Tell him he's real important to the company.... Words are free. Managers have no budget on nice-eties and things they may say to you when trying to praise you up into doing this.
More nefarious managers may even dangle a pay raise or promotion 3-6 months out into the future, which is also FREE - when the time comes for it, magically there will be an HR issue, hiring / raise freeze, etc. The words you hear from your manager in the context of convincing you to work more are almost 90% of the time BS.
Fail enough, and they will understand you are at your capacity and need additional help. As long as you are putting in a reasonable amount of effort, there's no chance this backfires on you. (Literally... I've seen companies struggle hard to fire someone in 3-6 months that was actively causing problems for everyone around them).
In the end, you'll be much happier.
That's not to say, don't do your work - but don't be afraid, don't be stressed out that everything they give you has to be done at the deadline they ask for it. 95% of the time, if something isn't able to be made by a deadline, you let them know, and they just report it up the chain. Most times the only thing they ask for when finding out it wont make it is just, oh, darn, well when can we expect this by then?
This is effectively why it seems that employees aged 40+ collectively can't or won't get many surprise requests done without saying oh we have to plan this etc months into the future... They are professionals who have learned properly how to manage expectations and keep a work life balance. The sooner you get a handle on this, it will have an exponentially positive affect on your work life.
Additionally - if you say you don't think you can do something in time, and then you work harder overtime for free to succeed, you've just discredited yourself. You're training them not to believe you in the future, because it worked out. Say you will try but may not have enough time to complete it with other things going on, then fail to get it done. This reinforces that you do actually know what you can and can't complete in a time period, and they will likely start to believe when you push back on things in the future.
Don't worry about the concept of being fired by the way... it's very expensive to hire (and train!) new employees. If you are doing an adequate job - and you would know very undoubtably well if you weren't - it would be a huge pain to fire someone just to try and hire someone else for 10-20% more effort, even 50%. There's potential legal liability / risk to firing without cause, and on top of that, the position would sit open for a month or two while they found someone else. It's very rare unless something you're doing makes it so terrible that they realize they'd be better off with the position empty for a while.