Eintracht Frankfurt midfielder Kevin-Prince Boateng says his departure from Schalke felt "like a punch in the face" with the clubs to clash in Wednesday's German Cup semi-final.
Boateng, 31, still carries bad memories after being released by Schalke in December 2015, six months after he was suspended over a "lack of mutual trust", as the club put it.
Boateng did not see eye-to-eye with Schalke's then-coach Roberto Di Matteo.
Now he has the chance to avenge his departure with a win at Gelsenkirchen in Wednesday's semi.
"It's the place I have thought the most about - it was like a punch in the face," he told sport1.de.
"It showed me the reality that everything is wonderful when it runs smoothly. And if not, you will be dropped.
"That was the (low) point for me.
"And I can be proud today that I came out of there and could turn all that power, sadness, energy and aggressiveness into something positive."
He said it had opened his eyes to how fast things can go in the football business world.
"Sometimes it's just better to split up, because then you can make a fresh start."
Boateng wants to beat his ex-club to put Frankfurt in their second German Cup final in as many years.
Bayern Munich face Bayer Leverkusen away in Tuesday's other last four clash.
"Schalke has shown this year that the team plays at a very high level," said Boateng.
"Even if it's not always pretty, they know how to bring home, or keep, the points.
"That's what makes them outstanding.
"I hope we learned our lesson that a 2-0 lead is not enough," he added.
Schalke, under current coach Domenico Tedesco, came from two-goals down to claim a 2-2 draw in Frankfurt in the league last December.
Boateng has been a driving force in Frankfurt's midfield this season after quitting Spanish side Las Palmas last August for family reasons.
After Schalke sacked him, Boateng rebuilt his career at AC Milan before a season at Las Palmas.
He has rediscovered his best form this season under Frankfurt's head coach Niko Kovac, a former team-mate at Hertha Berlin.
Frankfurt crashed to a 4-1 league defeat at Leverkusen on Saturday, the day after it emerged Kovac will coach Bayern next season.
"At the end of the day, we know that’s football -- it can happen," Boateng said after defeat dropped Frankfurt to seventh -- outside the European places for next season.
"We did a great job up to now and we want to finish the season the way we started it.
"We want to reach our targets and we'll see what's going to happen with the rest."
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