Exercise was one of the hardest habits to reinforce. There was a video that I saw (the trigger, behavior and reward method) that stated that I needed to know why I wanted to do it and how it made me feel. I noticed that I was more energetic, my muscles were less stiff and my knees hurt less from working out consistently.
A habit isn't necessarily formed out of obligation, it is formed out of the love of it. People love how food tastes, how tobacco makes them feel and etc. Whatever habit you decide to take on, make sure it's something that you love doing and would consider doing on a regular basis. Then, slowly work to getting there. You will get there. Even if you hop off for a little while, know that as long as you get back on, you will get there. Keep going and don't give up on yourself.