I believe myself to be a victim of my own mind, noticing thought patterns familiar to more than one destructive mindset, but the one mindset which seems to affect my success in life the most is the mindset of having my actions depend upon external circumstances. My personal ratio for reactive and proactive thoughts is probably about 10:1, respectively.
I often feel strongly discouraged and sad when things are not going as planned or as I would like them to be. Any sort of deviation from my perceived fortunate future - especially that which causes misfortune - causes me to worry a lot about both my happiness and the happiness of those around me, often times thinking to myself things like "If only I had a steady income from my freelance work..." or "If only I could maintain a car...". Thoughts like these, if not acted upon, slowly and painfully drain my confidence, something I wish to have more of in life. Hope without progress is just that, and lack of progress - or at least the inability to keep any kind of record of it - breeds sad feelings of hopelessness (brain memory, which is at least somewhat affected by emotions, can be unreliable - record keeping is a job generally more suited to a pen and some paper/computer with a keyboard).
I can't know for a fact that some people are genetically programmed to be happy and are simply less prone to worry, but there are countless success stories and living, breathing human examples around me that suggest that being mindful and worrying less about stuff that is out of our control or "outside of the moment" helps to gradually bring us closer to our goals and, in turn, closer to a state of contentment and happiness. I strongly believe we should not compare each others' quality of life but learn from our experiences (and especially mistakes), all the while staying as empathetic as possible and avoiding passing any judgement like it's gas.
Life should be an experience, with both sad and happy times, not something to waste by comparing it and worrying about it. I understand this painfully well, but too often do I take a passive role in my life both directly and indirectly, either when I am feeling overwhelmed by other tasks at hand or feel like I lack confidence in a given area of activity. When things work out on their own or when someone gives me a push in the right direction it feels great, but when plans suddenly change and/or something goes wrong, I get sad, blame both myself and others and ruminate on what could have been if only I had been more assertive and taken initiative in the past when I still had the opportunity to do so. Thus I have come to the conclusion that by changing and/or redirecting my way of thinking about decisions I make in life could improve my life experience - it won't necessarily make me more happy directly, but I will at the very least have more confidence in knowing that I made my own decisions when I needed to, using the best judgement I had at that moment in my life.
As my worries build up and swell my brain, I focus more on my past and future. As a result, I lose track of my current progress and become less and less able to focus on personal growth in the present, which in turn negatively affects how I feel and perceive my existence as a whole. I am certain that by taking steps, however small, to convert most if not all of my "If onlies" into "What ifs", I will open myself up to more opportunities in the present and keep myself from worrying about the things I have zero control over - what was before and what will be. The present is a gift to be cherished - our past is only wrapping paper and the future lies inside, possibly forming a familiar shape, but definitely uncertain.