Sopoaga turned his back on New Zealand rugby to accept a £1.5m pounds ($NZ3,063,512) three-year deal with Wasps.
In a candid interview with The Guardian, the 27-year-old said “coming over as a marquee player adds more pressure”, and he felt he had yet to reach his top form.
“It’s not that I don’t love this but there are things that do get you down,” he said.
“It’s not all glitz and glamour. A lot of the time people just see the 80 minutes, they don’t see what goes on behind closed doors and how winning and losing can affect players.”
Wasps – regular playoffs qualifiers – are fourth in the 12-team English premiership after five wins and four losses.
Sopoaga was signed as the marquee man to replace former England pivot Danny Cipriani, who moved on to Gloucester.
He is his own toughest judge and feels he is yet to reach the standards he set at the Highlanders.
“I take it all quite personally. I know when I’ve been playing well and when I haven’t and, at the moment, I’m just not quite there. It’s not through a lack or preparation or not trying my best. I know guys who have come over in different positions who have also found it hard but when you’re such a focal point …”
“If I came in as a fullback or a wing all I’d really need to do is know my moves, score tries, run fast and catch high balls. Here I’ve got to play chess and drive the bus. On occasions I’ve felt I’ve driven it well; on others I know I haven’t really nailed it.
“I feel that deeply and I shoulder a lot of that personally. Dropping balls or kicking out on the full are all things I can control.
“Coming over as a marquee player adds more pressure but there is no one with greater expectations of me than me.”
Sopoaga has had his eyes opened at the standard of English rugby.
“In New Zealand I don’t think we watch enough northern hemisphere rugby to truly appreciate it. It’s out of sight, out of mind. Because we don’t see it we’re like: ‘Meh, whatever.’
“Now, being amongst it, you’ve got some very skilled players and some pretty wicked talent. England have a very decent pack, some electric outside backs and a world-class 10. If you can get a fully fit England team playing with the mindset they did against the All Blacks last month they’re going to give themselves half a chance.”
But Sopaga is backing the All Blacks in 2019.
“They’ll come back stronger. I’d be 100 per cent confident that black jersey will be a very dangerous beast come the World Cup.”