If you want better luck making the exercise (or any other) habit stick better. Try one or more of the following:
* reduce the habit down to a ridiculously small subset of the habit. Eg. "I will floss every day" becomes "I will floss my upper, front two teeth". Or "I will exercise every day" becomes "I will change into my exercise clothes and step out my front door.". You don't have to floss all your teeth... just one. You don't have to actually exercise, just change and step outside
* put the habit trigger (your floss, your running shoes, whatever) where you have to trip over them during your regular routine. Eg. keep your exercise clothes on the chair where you sit to use your laptop or watch your tv
* put up a calendar where you have to see it every day (like at the dining table) and a red marker on a string (so you can't "lose" it) and make a Seinfeld chain
* enlist the aid of your family and friends. "Spouse... I'm doing this thing that's hard for me. I need you help. Please help me. And let's have a reward... if I [perform habit] for [reasonable time period... say daily for 6 weeks] I will take you to (or make it possible for you to) do [some activity spouse likes but doesn't get to do often]"
* have a big reward to work towards, but also lots of little rewards along the way to reenforce the good behaviour. Just make sure they don't undercut the habit
* make a consequence for failure. Something that won't really hurt you but will definitely sting. Make sure your friends and family know about it so you feel social pressure to go through it. "If I break the chain more than twice, I will publicly donate $1000 to [political party that I hate]... and the money will come from my budget for [hobby I love]"
* remove triggers for the habit you want to replace. Eg. If you want to eat healthy, then remove all non-healthy food from your home... don't hide it, don't put it somewhere out of reach so it will be inconvenient... get rid of it completely
* don't give yourself any wiggle room. Make your rules absolute. If you think you need an out (eg. might need to break $diet_plan due to business dinner) then explicitly give the authority to someone you trust and who has an interest in your overall success. Eg "I will strictly follow $diet_plan. My cheat day is Saturday when I can also eat [list]. Any other time, $spouse (or $friend) has to agree. Even so, I will stay within the following limits: $list"
* Plan out in advance how you will handle the curveballs that life throws at you (especially important to learn from the ones that caught you the previous times). How will you respond to $X? ... Decide now instead of hoping that future you will be able to detangle from the stress of $X enough to think clearly
* Take on only one habit at a time. Start the next improvement only after the current one is firmly entrenched.
If you have more then one, choose the one that makes the other ones easier
TL;DR; Assume you will be weak. Assume that it will be hard to walk the path of $good_habit and fall back into $bad_habit. Do everything you can now to set future you up for success by removing decisions, removing temptations, getting help from others, and making it more painful/embarrassing to give up than it is to simply do the new habit.