A person can easily be much more negative towards themselves than anyone else could. The human mind is built to try to solve problems, to notice when things are not right to remember negative outcomes. People have do a lot of very critical thinking in order to feel safe. Unfortunately this and lead to talking to oneself in a harsh way.
Accepting that this is true is the first step toward being nicer to oneself. Most self-criticism is overblown or unnecessary and is often the result of a lifetime of programming. For instance, if your parents said you were not good at math repeatedly, this idea will bob up in your mind every time any sort of transaction involving math comes up, which can be several times a day. Even if you are actually adequate in math, you would still habitually believe otherwise.
Learning to replace negative thoughts with positive ones is a process that can start immediately, but which usually takes time. This is ok - trying to change overnight often leads to relapse. Practicing letting go of one's negativity with rational, positive thought will eventually win the day.
Blessings, Cheryl Deaner, LMFT 36764