If you have trouble setting clear boundaries, it's likely because you're not saying a clear "no" or "yes" to things, situations, or people. They say that not making a decision is also a decision. Either it means a no, or it's decided for you.
Wouldn't it be nicer to have things under control yourself?
Often, it's important to make a decision at all. And don't be afraid, because most decisions aren't set in stone. You can revise, correct, or amend your decision after some time. The pursuit of always making the best decision with a tendency towards perfectionism is often more of a hindrance here. Make a decision and form an opinion that is consistent with the current situation and stand by it.
The Japanese principle of KAIZEN beautifully describes this path of continuous improvement - of oneself, activities, or processes. KAI = change, transformation, and ZEN = for the better. It describes an approach in which one constantly checks whether what they are doing is still the right thing. It's a cycle of the following steps: Observe - Plan - Do - Check - Adjust... and then it starts again with Observe. So: Observe the situation (or what it is about), plan, execute, and then adjust or correct what is not aligned.
Resistance: Must and should - when these words circle in your mind, it feels constricting and can lead to resistance. It overshadows an inner decision for or against something and can even cause stress. Choose FOR your own rules. When a belief arises, such as "but you must...", acknowledge it as a part that doesn't belong to you. Say to the thought, "Thank you for the warning, but I don't need you here." That's a clear no.
Anger: If you lack the necessary drive to say a clear no, try using the necessary energy - and that's anger. If something annoys you or someone tries to push something onto you, say a firm NO! No, damn it. You can practice this loudly at home and then convey it to the person concerned in a moderate but firm tone. Be wary of getting tangled up in explanations. No, unfortunately, that doesn't suit me, I don't want that, I don't need that... should suffice.
If you don't want to make a decision today about a specific matter, perhaps because you don't have enough information yet, at least write it down on a to-do list and set a deadline for when a decision should be made. Unfinished business without a plan leads to rumination and poor sleep. Committing to something also brings relaxation. And as I said, decisions can be revised. Keep asking yourself: Does this still work for me? Yes / No. Practice it. Practice, practice, practice... until it becomes automatic. Practice makes perfect!