going smaller

this is my second year of surfing, i went from the long board to a 7'' fun board. I can take good 5-6' waves if they are clean ... i find my fun board hard to duck dive (i pick the nose, but cannot go really deep), and i ve got the feeling it stops me to go out in bigger days head to head+3 at my beach break. i am 5'8'' for 170p. Should i go smaller or try to get better in paddling/duck dive before ?
If yes, what kind a board should i use as a next step ?
I ve got an old 6'9'' x 18'', looks really narrow to me (got pictures if it could help) - last time i tried it - 6 months or so, it was a nightmare .. maybe i need to try again or to find another board as a transition ? any advice ?
thanks

If yes, what kind a board should i use as a next step ?
I ve got an old 6'9'' x 18'', looks really narrow to me (got pictures if it could help) - last time i tried it - 6 months or so, it was a nightmare .. maybe i need to try again or to find another board as a transition ? any advice ?
thanks

