Your therapist seems to be totally discounting the possibility that these memories may be false, and that, I'm afraid, is not as realistic view as you have yourself.
The only way you can be sure, 100% sure if these memories are real is if you can have them corroborated by someone else, or find some sound evidence to support them. Failing that, there will always be a degree of doubt in your mind.
You will be aware, I'm sure of False Memory Syndrome, a situation in which people come to believe in the truth of 'memories' which are actually imagined rather than real.
In most cases, these memories relate to abuse during childhood.
It has to be said thjat this happens most commonly in people who have been in therapy, and often the idea of thse 'memories' is implanted either wittingly or unwittingly by the therapist.
Now please note that I am NOT saying that your memories are false - simply that you must remain alive to the possibility that they are.
You will find some sound, detailed information here
There is some interesting information here
Basically, it is possible for things long forgotten, or repressed can emerge into consciousness, bu establishing the truth of them depends on other evidence.