I went for the first time but I didnt have the expereince I hoped for

I used to take swiming lessons and I used to be good at swiming which is an advantage , it has been an while though but today the sea was really bad and I didnt have an very good experience so I cut my time short. There was an red flag warning but I didnt know what it was , I thought black meant bad luckily someone told me so I got out quickly. An guy tried to help me which was nice but he had to get out too as he said it was too dangreous. I did tell the woman I rented from I have no experience , I feel so embrassed and horrible , I wasted my money and I shown myself up but At least I was brave enough to try it. I also bashed my knee on the rocks. Its an pebble beach and its black because of the volcano errupitions. Any advice on how to get confident enough to catch small waves and stay in a lot longer safely , please let me know. I didnt realise it was so hard , Also I have very strong muscles from gymnastics and pole dancing if that helps any. Im posting this in confidence as I feel really bad about what happened.