Keep riding, give it time and focus on longer sustained efforts instead of short hard efforts until your body acclimates to the lower air pressures.
I lived at ~ 7000' for about a decade and during several of those years worked and trained for up to a month or more at a time at sea level. The first few weeks back at altitude after longer trips were pretty tough while out on the bike but give it time and your body adjusts pretty well. The bad news is that even when fully acclimated your sustained power at altitude will be a bit lower than what you could do at sea level, but the good news is that it's easy to go fast and sustain higher speeds in the low density air up high and other riders have to deal with breathing that same lower pressure air so it pretty much levels the playing field.
Just keep riding and don't beat yourself up too badly while your body is adjusting to the altitude and you'll do fine. Pay extra attention to hydration, you'll lose a lot of water through respiration in the drier high altitude air and it catches a lot of folks by surprise.
-Dave