Jessica Simpson Performs Live for First Time in 15 Years at South by Southwest

Jessica Simpson couldn't deny the "Irresistible" urge to get back onstage ... 'cause she just had her first live performance in more than a decade -- and, she says she did it to remember what really inspires her.

The singer-songwriter and actress hit South by Southwest -- the popular film and music festival in Austin, Texas -- performing at the Recording Academy's Austin Chapter Block Party ... and, the event marked her first performance in 15 years.

JS posted a clip before she hit the stage ... saying she felt excited and blessed to be performing again after her long hiatus.

When she jumped onstage, Jessica said she finally got back to performing because she needed to get back in touch with who she is ... and "forget who they told" her to be. Unclear exactly who "they" are.

Jessica mixed some of her classics with songs off her upcoming album ... including her track "Breadcrumbs" -- one of the songs she's teased from her "Nashville Canyon Part 1" EP which is set to come out later this month.

She also paid respects to her hit cover from 'Dukes of Hazzard' ... dancing around to Nancy Sinatra's "These Boots Are Made for Walkin'" -- a crowd-pleaser in the Texas town.

Jessica's performance comes amid speculation her new track is all about her split from her ex-husband Eric Johnson ... including a line which led to fans to conclude the former NFL player cheated on her.

The following line from her new song "Leave" which has stirred up all the rumors ... "What we had was magic / Now you made it tragic / Giving her what you gave to me / Now the well that you drank from’s empty.”

As we told you ... Jessica and Eric officially split back in January after 10 years of marriage. Simpson told us the two had been "navigating a painful situation in our marriage. Our children come first, and we are focusing on what is best for them. We are grateful for all of the love and support that has been coming our way, and appreciate privacy right now as we work through this as a family."