And after belatedly linking up with the Lions, Best will now have the opportunity to stake a claim for a Test jersey when he packs down at hooker against Western Force on Wednesday (11.00am).
"My emotions ranged from the two extremes," said Best.
"To miss out on the initial selection was bitterly disappointing but I had to try and get over it a bit quicker than if Ulster hadn't been involved in such big games.
"Obviously there was the disappointment of losing the PRO12 final with Ulster, then the next day going to the complete opposite extreme of getting the phone call to say they want you in London on Monday morning to fly out to Hong Kong.
"I don't think there has ever been a higher high than that for me in rugby. This is the opportunity of a lifetime.
"Competition for places is what it's all about. For me whether I was selected in the initial or coming in late as I did, I'm here and I'm getting a chance to play.
"It's a massive honour for me to be playing tomorrow."
Best has swiftly got up to speed with Britain and Ireland's elite and reckons he can take confidence from a host of players who have gone from late call-ups to Test starters on previous tours.
But he is well aware of the standard set by those who pulled on the famous red jersey in Saturday's 59-8 victory over the Barbarians.
"Yeah it definitely does. There are 37 players in the squad and all of them want to play in the tests and that's the way it has to be and that's good," said Best.
"It breeds success and that's where we all need to be. Everyone will get a chance and it's up to you what to do with it.
"There has been pressure put on all of us that arrived later to make sure we get up to speed.
"It's not a new thing; having played international rugby for a long time you come in and you get about a week to prepare and then you're into international week. From this side of things it has been a little more condensed, and a lot more pressure.
"The general template isn't new, but everything is there for you and the coaches are brilliant in terms of going out of their way to make sure you know exactly what's going on.
"Few people get to do it and you want to get out there, do your best and push to be selected for the Tests and everyone has the same mind-set for that.
"Obviously it's very exciting to be getting a start tomorrow night but it's also nerve-wracking. The bar was set high on Saturday, and you're not going to get many chances.
"There's only ten games and everyone needs to get a go, but I think from my point of view it's about knuckling down or trying to be as good as I can be, and then whenever you're not playing support whoever is playing in your position and support the team. That's what a tour is all about."
The Lions expect to face the full brunt of the Force in their first match on Australian soil and Best has echoed the sentiments of Gatland and forwards coach Graham Rowntree.
"Discipline's key when you get to big games and you're playing at the top level which playing for the Lions is," added Best.
"Three points and penalties are massive and if you concede them you're going to be in trouble. If we have a penalty and somebody retaliates and it's reversed, things like that can change games, can change momentum of tours even, so that's where we need to be.
"We don't intend to take a backward step but we need to be disciplined in our approach to that. Everyone knows you can be physical without going over the top."