Sean burst onto the scene last summer under former coach Matt Williams but had mixed fortunes in the autumn Tests and this year's RBS Six Nations Championships.
Sean keeps his place in Hadden's side to play Romania on Sunday, when he will earn his 13th cap, but the winger admits he was unsure how he would fare under the temporary boss in Scotland's final two matches of the season.
He said: "I am more comfortable now than I was in the last autumn Tests, which were very bad for me to say the least. My form picked up through the Six Nations and so it would be good to carry that on against Romania.
"Matt Williams showed faith in me for those Six Nations games and it is good that Frank has also showed his faith in me. Obviously that helped me and now I'm settled.
"It's always nerve-wracking when another manager comes in especially for someone like me who doesn't have many caps. It would have been easy for him to let someone else have a go. I was nervous before the Barbarians game and then I was okay once I knew I was in.
"Obviously Frank is a new manager and you don't know what he's thinking but I was very relieved and happy that he stuck with me in these two games but hopefully I'll cement my place in the side in this game in Bucharest."
After beating the Barbarians 38-7 at Pittodrie last week, Scotland hope to finish off an otherwise poor season with an encouraging win in Bucharest.
And Sean accepts defeat would be unthinkable against the unfancied Romanians.
He said: "We must get a win over there. It would be awful for us to have won last week so well against the Barbarians then to go over there and lose, even by just one point. Training has been very enthusiastic and very good fun but we need wins.
"It is another cap for me and I just want to keep playing rugby and honing my skills at this level. Scotland have lost twice out of three away games to Romania so I want to make it two-all."