Well, you forget everything you learned because you lack mindfulness. Loss of mindfulness is precisely what you described: forgetting the teachings.
You can't have your path confirmed. Advanced practitioners can tell you there opinions, but not confirm your level of attainment.
To define stream entry and the way it occurs is a rather arbitrary process, but ...
Im not sure about guarantees but there are these relevant passages;
Association with people of integrity is a factor for stream-entry.
Listening to the true Dhamma is a factor for stream-entry.
Appropriate attention is a factor for stream-entry.
Practice in accordance with the Dhamma is a factor for stream-entry.
— SN 55.5
There is the case ...
Ānanda brought a similar question to the Buddha, attempting to hedge and bridge the issue of solitude and social interaction:
SN3.18:3.3: ‘Sir, good friends, companions, and associates are half the spiritual life.’
But the Buddha would have none of that and corrected Ānanda immediately:
SN3.18:4.2: ‘Not so, Ānanda! Not so, Ānanda! Good friends, ...