Because you learned different languages before trying Spanish, it means your brain is already ready to learn a different language. it should not be too hard for you because you already learned Greek, and a lot of the verb formations and verbal stems are the same, also you will see that a lot of the words are similar to the Spanish.
The best way for you to learn would be going to a country that speaks the language, and stay there for maybe a year or so. However, I know that it is not an easy task to move out of the country, so here are some tips that will help you learn better:
1) Write the name in Spanish of objects around your house on post it notes, and place them around. Every time you need to do something or use something around your house, force yourself to say what you need in Spanish.
2) Make some friends that speak Spanish. The more you are around people that speak the language, the easier it gets to learn.
3) Don't be afraid of making mistakes. If you make a mistake, ask your Spanish friends to correct you, they will feel honored to help you.
4) Go to places where you will see Spanish. Go to a Spanish church, go to Little Havana in Miami, if you can, go to Mexico, Spain, Colombia, Puerto Rico, or any other Spanish speaking country.
The main point of it all is to come out of your comfort zone. If German, Japanese, Arabic and Greek were easier to learn, it is because you have the affinity to learn those languages, in other words, it comes natural to you. To learn a language that will require some sweat, you have to change your game plan.
I can say it by my own experience because I could never learn English in my country (Brazil) before I came here. When I started to learn here, I literally had pain in my head every night, but if you persist, there will be a time that you will tell yourself you will never learn that language, and that is when you start to learn.
Henrique Gomes (Portuguese, English, Spanish, Greek, Hebrew)