Hello and a belated welcome to the forum.
Meditation can seem difficult because we often lose our concentration. I have been meditating for years and still have problems. Some days are great, others not so good. When you realise you are getting distracted it can be difficult to get back on track so to speak. Waste no effort on trying to put something out of your mind by willpower. Instead focus on the meditation. Try to acknowledging the interruption and say you will get back to that thought later. Be gentle with yourself and start meditating again.
It is easy to get annoyed and dispirited. We are not used to calming ourselves in this way, not used to reaching our calm centre. While our brains are very useful they do like to retain the status quo. Meditation is a huge change in managing ourselves and the brain does not like change. it will happen but not overnight.
How do you meditate? Filling your mind with pictures etc is just distracting in my opinion. How are we supposed to relax while we frantically try to keep a picture of the sea in our minds. I use a mantra which I repeat silently in my mind and that's what I concentrate on. Yes I find myself dealing with other thoughts before I realise it but when I do realise I put the thought to one side and gently bring myself back to meditation and my mantra.
I hope that helps. Love to hear from you again.