Sylvie, since this is the first time you've ever worn booties & OH fins, there really will be an adjustment period.
I recently moved from OH to Full-Foot... and it was an adjustment for me as well.
But you'll get used to it... it really is very different from full-foot fins since there is actually pressure on your tendon... unlike full-foot fins which slip on and feel like socks.
Actually, the holy-grail of OH fins is to make them as efficient and as comfortable as full-foot fins... since you can only use FF fins in warm waters.
Sylvie, just a thought... Mares makes booties that fit well with their fins (surprise, surprise) ... the soles of their booties grip the inner ribs better... and the back heel offers better protection against the heel strap.
I use their SeaBoot and I found that it does work better than most booties I've tried due to the extra heel protection... and I can wear my fins strap against my heel & not my tendon, and it grips a lot better since the heel protection has rubber grip specifically to grip the heel strap. I don't even have to tighten my strap as much anymore, so the heel strap is almost just resting on my heel... and not being stretched! This goes for all fins that I use with my Mares booties.
I didn't think it would make much of a difference... I just figured that I'd give them a try since people rarely think that different style booties make a big difference... well, for me... they did.
Check out the Mares Seaboot or the Avanti boots (The Avanti boots are supposedly the better ones) They are the ones with the heel protectors. The Avanti boots supposedly have better on-land grip..blah blah blah