I have been meditating for years and can relate to your frustration. Some days are better than others for meditation. As others have stated, don't take it too seriously. Meditation is like working out a muscle, the more you work it out, the stronger it gets. Try and not fight the mind. Allow it to do it's thing. The more you try and clear thoughts, the more thoughts will come. What you resist, persists. Go with the flow of it.
What works for me is Zazen meditation. Just sit. No rules. If you need to scratch an itch, do it. If you need to move around a bit, do it. Don't beat yourself up over not doing it right. It is just sitting, nothing more. Sometimes I will rock back and forth a bit. Sometimes I will chant if I feel like it. I like to count beads on a 108 bead mala when I want to work on a certain mantra or chant.
I don't worry about concentrating on breathing most of the time. I find the breath takes care of itself and will slow on it's own. I press my tongue to the roof of my mouth gently, this reduces saliva build up in the mouth and will result less swallowing, it also helps to open up nasal passages for better breathing through the nose.
If I feel something, I try and feel it all. I go into it. I allow it to play out until is passes or until my time is up. Thoughts will come and go constantly. This is the time to observe how the mind works. Watch as it creates thought after thought after thought. Don't be irritated with the thoughts if you can help it. Try and laugh at yourself as much as possible.
Another thing that has helped me is to smile really big before and I put my hands in the air like a victory. This can prime me to be in a better frame of mind for my meditation. Sounds silly I am sure, but it works for me. I also like to light incense during my meditation. I highly recommend a good zafu and zabuton! This made a world of difference in my meditation practice.
Not sure if any of this will help? Stick with it and practice every day, at least 15 mins. It will get easier and become more fulfilling. I find I really miss meditation on days that I skip for one reason or another. It has become something I very much look forward to. It is very important to me! I made it a priority in my life.
Keep in mind that you can meditate anytime, anywhere. Driving... you are driving.. nothing else.. concentrate on driving. Any activity.. try and be present.. be present as much as possible and meditation can occur all the time. Being present is the point of it all. Be here, now!
Good luck to you! I hope you stick with it!