5 year old letter help

I feel like a failure. My 5 year old doesnt know her letters and the school year ends in May. I feel like other kids know this stuff but she just doesnt. Preschool has just recently gone back to in person after being virtual since November (before that they were only in person for like 2 and a half months). When I try to help her by using flashcards I cant help but get frustrated when she doesnt pay attention or just says "I dont know". I feel like I gave her too much grace the rest of the year. I just kept thinking "Oh we are in a pandemic. Its fine. She will catch on." but now she is really struggling and it breaks my heart that I let it go on this long. Any ideas on ways to make it fun for her to learn her letters and the sounds that they make?