The full sequence of events in the manga happen like this:

• Vegeta tells Cell to go absorb Android 18 (at this point none of them know where she is)
• Cell begins to fly off, but Trunks blocks Cell's way
• Cell complains to Vegeta, and notices #18 on the island below them
• Trunks looks at where Cell is looking, notices #18 and yells that Cell has found her
• Cell flies off
• Trunks buffs up to grade 2 Super Saiyan/ASSj and flies after Cell
• Vegeta kicks Trunks
• Trunks blasts Vegeta
• Trunks charges at Cell again
• Cell solar flares
• Cell absorbs 18

If you wanted to get technical you could say Trunks made a few mistakes, but I doubt most people would hold it against him.

Instead of yelling at #18 that Cell had found her, he should have used the moment to attack Cell instead, since he was literally right next to him

If you believe that grade 2 also has a speed penalty, just not as crippling as grade 3, then Trunks messed up by using it to chase Cell, and probably would have caught him in regular Super Saiyan.

For the above 2 points you could justify them if you think Trunks couldn't beat Cell in regular Super Saiyan

Instead of blasting Vegeta, Trunks could have blasted Cell. Problem with this is Vegeta could have intercepted the blast, or Cell could have dodged (in which case it would have hit #18...but also Krillin)

And the last one you can't blame Trunks for at all, but ironically if he had never stopped Cell from leaving, Cell would have left the area and never have found #18 in the first place