I think the trouble with meditation is the perception that the mind has to think of nothing. This is possible but only by extremely developed minds that have practised meditation for a long time. To most of us meditation isn’t so much blanking the mind as it is focusing the mind.
I think where you are going wrong is trying to force yourself into being calm and this in turn is making you agitated. It’s a bit like trying to sleep and then panicking that you should be asleep which makes you stressed and therefore more awake. What you need to do is find a meditative style that works for you and this needn’t be the same type of meditation every time. Different moods will product different meditative states.
I find that there are two stages of meditation, the shallow and the deep. My shallow meditation is just allowing my mind to wander and do its own thing. I am not particularly focusing on any subject, I’m just allowing words and thoughts and memories to flow through my mind and wander where they will. I think this is a really important stage in learning to meditate. Just make some time to sit quietly with your eyes closed enjoying your own thoughts. If you can stop the pressure that you feel you are doing meditation wrong you will start to enjoy the experience of taking that step into your own head and the more you do it the easier it will become to switch into a relaxed meditative state.
I would suggest you that you start by allowing the thoughts to come into your mind arbitrarily but that over a period of time you start to bring thoughts into your mind that you want to use as an aid to your meditation. Different things work for different people. My whole meditation ritual is focused on visiting my Grandma’s old house within my mind. I rock in her armchair, I lie on the back lawn and absorb the rays of the sun, I dance in the living room during a thunderstorm. These very specific focused thoughts allow my mid to be freed from all the clutter of the everyday mundane world. This is the beginning of deep meditation. As you sink into your own mind you can start to experience the sights, sounds and smells of the world of your meditative state.
That particular idea is something that works for me. Other techniques are to meditate on a thought - popular with Christians who may choose to meditate on a specific passage from the Bible. You may choose to try holding something very simple like a colour in your mind and focusing on nothing except that colour. I’ve known one person who meditates by sinking deep into herself to find (what she terms) her joy and then the whole meditation is devoted to holding and embracing that joy.
The point really is that there isn’t one way of meditating. Work with different ideas and experiment with what works best for you.
On a practical basis I think it is really important that you go into meditation fairly relaxed to start with. A bath or a walk in nature may help to clear your head and get you into the zone. I have tried coming in straight from work and sitting straight down to meditation and it doesn’t work well for me as it is so hard to switch yourself off from the events of the day. Suggest you distance yourself from the day first. A good way to do this is to write down everything that is currently on your mind before you start so you can free your mind from it and move onto the thoughts you want to focus on during your meditation session