I checked in morning to see if will be able to make “hefsek teire” that afternoon. It slipped my mind and I fell asleep and woke up when it was already night. Could I rely on morning bedika even though it wasn’t with intention of it being used for the actual Hefsek?
Provided you did a full bedikah in the morning, you can rely on this for a hefsek tahara.
It is preferable to do the hefsek in the afternoon, and even if a bedikah was performed in the morning, one should return to do the hefsek in the afternoon. However, if this was not done, the bedikah performed in the morning is fine, and can be relied on.