Here are the top 5 commons mistake I've seen from people in their fitness journey.
Doing too much too soon
Not having a plan
Wanting something that feels challenging
Obsessing about the outcome
1. Doing too much too soon
So you decided to start working towards a new goal and you feel pumped about it. You start to think about all the things you have to do and all of a sudden they start to pile up. Eat better, grocery shop, meal plan, workout, hydrate, sleep, maintain a social life, recover, water the plants...the list goes on. Try to do everything at once and you are sure to end up overwhelmed and fed up with this new goal. You may be able to maintain this for a few days, maybe a couple weeks but I've rarely seen people who try to do everything at once succeed for much longer than that.
This leads to the yo-yo effect, of doing everything perfectly for a few weeks, then just stopping everything, only to start again once you feel like this time it'll be different.
When you start, take things one at a time and give yourself the time to get good at that one thing before moving to the next one.
One way you could do this is being consistent at showing up for your workouts. Keep the rest of your life the same, and just get good at showing up. When you do that consistently, pick something else like grocery shopping, or sleeping better, or portion control. Then get good at that before adding a third thing.
2. Not having a plan
A plan is a general outline of what you are going to be doing in the future. It covers what you'll be doing in the gym but also has contingency measures when life gets in the way. What will you do if you miss a workout, or miss a meal plan day? These are answers that a good plan takes care of. After you do your first two or three weeks when you are motivated, what is the next step?
When you start a fitness goal, map it out 2-3 months into the future. What kind of workouts will you do? How will you progress during the next few months? What will you eat? When will you be able to have cheat meals (The answer should be at least once a week)?
When you have the next 8-12 weeks mapped out, start planing each week when they come. What days will you work out? What happens if you miss a scheduled workout? (You should have back up days for missed workouts). What day will you go grocery shopping and what days will you meal prep?
A good plan takes care of all the "what ifs" that may arise at any point during the next few months.
3. Being impatient
Wanting your hard work to yield results is natural. Wanting it to happen in a week is problematic.
Your body needs time to adapt to the changes you are putting it through. Often time the big changes require lots of small incremental changes beforehand and you may not feel these changes instantly.
Example: You want to lift heavier weights. You will most likely need to go to the gym and work on your technique first, then gradually increase the intensity of your workouts and then, finally lift heavier weights. The initial changes, of working on the technique are very subtle, and you may not feel them right away. They are nonetheless necessary steps to lifting heavier, which is what you want to see.