Cycling – I’m with a group and these are rollers. It’s hard to have a slower pace to practice correct breathing.

Two possibilities here:
1. Bite your pride and shift gradually to lower gear keeping your mouth shut. Then evaluate your aerobic capacity after 1 or 2 weeks of doing so.
2. Shift to mouth breathing only when possible (not uphill) – but my qualified guess is that it will take much longer time.