A lot of people in this thread appear to be pretty hard on themselves. There seems to be a trend of people who want to be nice, are trying to be nice, but don't see themselves as nice. If that sounds like you, then I've got some good news for you:

You are a nice person.

If you're sincerely making the effort to be a better person then that's admirable. Self improvement is hard. Too often people are quick to judge based on the result of your actions rather than the effort that's put into them. To put it another way, we judge people by their actions but judge ourselves by our intentions.

Treat yourself to the niceness that you're trying to show to other people. You're doing the best you can. You're trying to be a nicer person which means you're trying to grow. From tiny seedlings grow mighty oaks, and the seedling shouldn't be shamed for starting its journey. Rather, it should be encouraged to keep growing.

If you find it difficult to be nice, but you're trying to be a nice person, I'd say that's a lot nicer than being the person who dismisses another for not being 'nice' enough.