8 is too big for you with only 65kg.
Try Exocet Fusion 7.0, very light with only 3 battens ... or even 6.5 to replace your old arrows sail... A guy in Portoroz is selling them for 250? (new sails)....they are quite strong sails...If you want additional information, call Mr. Alan 040633999...he seems very arogant on the phone, but don't be freightened, he is an ok guy, he can transport any stuff to Ljubljana, if you find it closer...he also sells a lot of equipement at low price, older models...
www.exocet-original.com/I think technique is the most important issue here, you said your boom was set very low, this needs to go up to your shoulders at least. It also depends how you rig your sail...you don't have time to surf on weekends? 1 day trips and you can get to stronger winds to expand your knowledge and see the progression ;)