Never say anything negative about yourself.
Who you think you are is only your perception of who you are. It is not who you really are. This perception can be changed quite easily. You can learn to stop having negative thoughts about yourself.
When you catch yourself having negative thoughts, do this:
Try to find specific instances when you disproved the negativity that those thoughts represent.
For example, if you think you are a slow learner, think of instances where you knew the answer faster than others.
When you cannot change an aspect of yourself, find a way to view it as an asset.
Even slow learning can be an asset if you find something that you gain by having this characteristic.