Keep in mind too there are ways to practice, especially with Amitabha, that don't involve lengthy sadhanas.
My own experience has been that doubts, like you say, roll in and I have a hard time.
If you don't feel like these doubts can be worked with (and how precious to see your obstacles right away!), why not try the most basic mantra of the particular deity?
For me, "namu Amida Butsu" or even just "Amitabha" is enough.
When I was working with Chenrezig, it was "om mani padme hum".
The short ones you can recite to yourself during everyday activities.
And in my experience, those most ordinary, least when you expect it moments are the most important.