How do you overcome getting over someone and not forming old habits I'd overcome in the process?

Hi - thank you so much for reaching out with your question.  Breakups are difficult for any reason (even if they are from short term relationships), so I imagine it is so hard to adjust to this unexpected breakup since you were in a long-term relationship.  You have put in a lot of effort, time and emotion, and it is hard when things don't work out in the way that you had hoped.  It is understandable that you would be concerned about slipping back into old habits and negative patterns since you are going through this and since things can go awry, particularly when feeling stressed or overwhelmed by difficult emotions. 

I think you have already taken the first two steps to addressing this issue, and that is being aware that it is a possibility and reaching out for help.  You are in the right place!  Here are some other things that could help:

1.  In being aware of what could go awry now that your emotions are raw, it will be important to potentially list for yourself all the things that you have been successful at overcoming and managing.  Increasing awareness of this, as well as any known triggers that you can think of, will ultimately help in addressing any temptation to slip back into old habits.  Keeping a list of this (on your phone, in a journal, etc) will allow you to refer back to your list when things get difficult.  It will be easy to act out of emotion at this time because you are vulnerable, hurt, angry, sad, etc.  Being aware of any potential pitfalls and checking in on your list will help to be able to more effectively apply the rational part of your brain to reason with yourself should you begin to struggle.

2.  Choose a strong, supportive accountability partner.  This could be anyone who you trust and who could serve as a sounding board for you, an encourager to make positive choices, and who will also challenge you if you begin to lean towards those old habits.  This could be a friend, family, pastor or therapist.  In any case, you want someone who you will listen to and respect when you are feeling weak.

3.  Write in a journal about all the reasons that you quit the unhealthy habit in the first place.  This will serve as a reminder for you that you overcame the problem once, and you can manage it.  If your partner was a support for you, that's okay - you can find another support, even if it isn't a romantic partner.  Remind yourself it doesn't mean that you have to backtrack to the person you were before the relationship.  

4.  Increase your awareness of any negative self-defeating thought patterns.  Once you are aware of them, challenge them with true statements about yourself and reframe those negative thoughts.  This can be difficult to do, especially when encountering difficult emotions as well.  It may be helpful to reach out to a trained professional if you find that this is too difficult to do on your own.  Cognitive Behavioral Therapy is an excellent evidence-based model to address difficult thought patterns which potentially got you into the negative habit in the first place.  

5.  Reach out to a trained professional if your functioning in any area should begin to be negatively impacted (too much/too little sleep, too much/too little food consumption, difficulty maintaining job, school or home duties and responsibilities, addressing problems with substances, etc.)

I hope this was helpful.  You will get through this, and you will heal.  It may be difficult, but you conquered those negative habits previously, and you can apply that same strength to stop yourself from engaging in them again.  I wish you well!   You got this!