Your mind wandered during prayer? Walk it back. That walking back is part of praying.

Attention wanders, and noticing that it has is not the failure. Returning is the practice. Nobody holds a thought still for ten minutes, so a prayer is less a steady beam and more a series of small walks back, and each walk back is the work rather than evidence of your unsuitability for it. The scolding you add on top is the only wasted part.

When you catch your mind in the parking lot of a totally different subject, just come back. Skip the commentary about being bad at this and pick up where you left off.