Let us see. First and formost you need to study the basics for a while. Usually people recommend you study the basics for a year and a day but from my point of view, you commit yourself to it until you feel a little confident. Not all people are the same, so it might take less time to some than to others. Secondly, you say you have cast several spells over the years. What kind of spells were these spells? If they defy the physical laws they are not going to work. Thirdly, when we cast a spell we don't just mix herbs and chant ancient words just for the sake of it. No. The words ,and the ritual and the tools that you're going to use simply help you concentrate and stay focused. You must really want it, you must feel this with every fiber of your being. You must know that the spell is bound to work. You mustn't have any doubts.
Doubts can only bring failure to your magickal working. Are you physically and spiritually prepared before the spell? People usually like to do a cleansing ritual before any kind of magickal working. When you pronounce the words from your spell, you MUST feel them with all your power. When you're casting a spell you stop thinking about everything else and you simply concentrate only on the spell. Do not allow anybody and anything to distract you and create a "magickal atmosphere" as I like to call it. Things like that usually help people alter their state of consciousness and focus on their spell. You need to be persistant. If you have cast the same spell several times and it still doesn't work ,maybe it's just not meant to be. Maybe it's just the Universe giving you a sign. Take the sign and try to see what it means. Also good visualization is required.