The center part of my lower lip split and is bleeding nonstop for 20 minutes with pressure applied. Whatshould I do?

See a doctor. I would see someone about this. You need to get the bleeding to stop and find out why it has continued to bleed despite pressure.
Bleeding lip. When bleeding does not stop after 10 - 12 minutes of sustained pressure, then other possible explanations such as a severed vessel or poss ile bleeding disorder may be the reason. In the former, a minor surgical procedure is required to control the bleeding. In the latter surgery as well as blood products may be necessary .
Need more info. I will assume this is an injury to a child. It would be unusual for it to bleed after 20 minutes of steady pressure. Most people hold pressure for a moment and then peek, hold pressure again for a minute and peek, etc. This is not 20 minutes of pressure! hold steady pressure for 10 minutes without peeking! if fat is showing or the bleeding does not stop, a stitch or two or glue may be needed.