How can I stop feeling sorry for myself?

That all depends on why you feel sorry for yourself.

If this is a short term thing, you can usually wait it out. A lot of people just feel down once in a while.

If there's a specific reason that you feel depressed, you can work through that. An injury, stress, break-up, or anything like that would fall under this category. Either directly addressing something that's in your control (therapy for an injury, change of routine for stress, etc) or indirectly (taking up a hobby or reconnecting with other friends for a break-up) may give you a bit of a boost and get you back on the right track to healing.

If it's a long-term thing that has no reason that you can find, you should consider talking to a doctor. Depression beyond the normal ups and downs of life may not have an answer that you can get to on your own.

Regardless, you may want to try a few little things that can help temporarily:
Take up a hobby. Something to occupy your mind and give you a bit of focus.
Change your routine. Drive a different way to work, rearrange your furniture, sleep on a different side of your bed (I'll switch to sleeping with my head at the foot of the bed if I'm feeling a bit off for a few days).
Get healthy. Change your diet, start exercising.

To be able to help you better, may we have a little more information on your situation.

In addition to the great posts here, I would only add this...

It may be that you have created some kind of pay off to it. Look for how it "works" for you (as weird as that sounds) and you may be able to discover what it is and then redirect your energies into something more constructive than throwing yourself a pity party. That gets pretty lonely!

It can also be that you've made a habit out of it and will need to put something specific in the place of accepting your own invitation to the party. Something like every time you catch yourself talking that same old trash in your head, you will then think about something constructive for the next xx minutes. Make yourself a "gratitude list" in advance so you can pick something off it! Or make it so that you have to add one more thing to the list even! The first item on your list could be "I know how to ask for help and I get it too!" :p

Amazing how many blessings come our way when we have gratitude. I found the most healing place to cheer up is being around nature. Remember to take care of yourself and your soul.