First you should make sure that you don't have a small selection that you are painting outside as you said because it is the common mistake. In paint brush, make sure that all setting in the option bar are good or reset it. In clone stamp, you need first to press Alt+Click the area that you need to clone then you can press click and drag to clone from.
In eraser tool and the all above, make sure that you select a regular layer not a smart object or locked layer in the layer panels. You can also provide screenshots to be able to help you more.
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