There are many professionals who do career assessments for a living. For professional help, search for an Industrial/Organizational Psychologist in your area; many I/O psychologists work as consultants and will do individual career assessment. If you do not have access to an I/O psychologist or aren't quite ready for that step yet, your best resource for help changing careers is putting your own self-knowledge to use and doing an informal career assessment yourself. Career assessments generally fall under three categories: interest, ability, and personality. One you know what you enjoy (interest), what you are good at (ability), and how you consistently react to your environment (personality), you can get a pretty good idea of the career path that will work best for you.
One theory about career interests (Holland codes) defines six categories: Artistic people thrive when they can express their creativity;
Realistic people need to do hands-on activities where they can see the end result of their labor; Investigative people are at their best when they are answering questions; Social people like to help and engage others; Enterprising people are good at entrepreneurial pursuits, such as starting businesses or developing new products;Conventional people need a lot of structure and may like to work with numbers, patterns, and routine.
Most people have a top two or three categories, which can combine to match certain career choices. For example, someone who is both Enterprising and Social may align well with a career in real estate, where they can help others and also having control over their schedule and workload.
Look for "Holland codes" online for many more resources about this theory.
The most general ability assessments are IQ tests, which give you information about your intellectual capacity. Although relevant to almost every career, you may want to look at more specific abilities. If finding your interests (above) helped you hone in on some possibilities, think about the abilities needed in those careers. Abilities can range from abstract reasoning to manual dexterity. Take into consideration whether you can speak multiple languages, can play a musical instrument, or are good at verbal or written communication.
A good framework for thinking about ability is provided by Howard Gardner's Multiple Intelligences Theory, which includes eight types of abilities: logical/mathematical, verbal/linguistic, visual/spatial, bodily/kinesthetic, musical/rhythmic, naturalistic, interpersonal (good at connecting with others), and intrapersonal (knowledgeable about the self). Taking time to identify and recognize your strengths and weaknesses in these eight areas may be helpful. Take note of where your abilities may overlap with your interests. For example, if you are interested in Social pursuits and also have high interpersonal ability, you can start to get a clearer picture of careers that you may enjoy and be good at.
Personality is a collections of traits and tendencies that make us who we are. Psychologists believe there are five main traits that make up the core of most personalities: Extraversion/Introversion, Emotional Stability/Neuroticism, Conscientiousness/Irresponsibility, Agreeableness/Hostility, and Open to Experience/Traditional.
There are many unofficial online assessments of these "Big Five" personality traits. It's important to take personality into account, because taking a career that goes against the grain of all your usual tendencies and patterns is exhausting and will lead to burn-out quickly. Along with your interests and your abilities, personality is the final piece in the puzzle to finding a fulfilling and prosperous career path.
To get the best help changing careers, all three aspects (interest, ability, and personality) are important to self-assess. A career that suits your abilities and personality is not going to be fulfilling if you have no interest in it. In addition, interest and personality will get you nowhere if you lack the ability for the job. Take the time to engage in self-reflection, using online tools and perhaps a professional, and you will be on your way to finding a new career that fulfills all your goals.