Please help a surfing beginner / loser

I have always wanted to surf, had few lessons in my life and decided to "go on my own".
My gf has found a good deal - the Linden Rio 7'2 board for $100 and we decided to buy it. I did a bit of a research and apparently hybrid fish boards can be good for beginners....
Well, I just returned from surfing and the board keeps sinking under my weight, I can move to the back but then I cant even reach the board with my bent legs (the chicken wing pop up technique).
I can't find the girth and the width but its this board:

I am 5'10 - 175-180 pounds, athletic, snowboarding experience (if it matters at all).
Do you think that I should keep trying, persevere and learn or this may be too much for me and I should sell it and buy an 8 feet fun board?
I would appreciate your help..
All the best,
Stanis