Welcome to the forum.
Since you have a history of losing 6 stone - you have proven to yourself that you can do this - and what we have done before we can do again. You certainly already will have acquired a good knowledge of how to lose weight healthily by eating correctly and by becoming more active...
I am sorry to hear about the broken relationship. It is natural to feel miserable at such times - and many of us are inclined to comfort eat when times get tough...
The good thing is that you havent regained all your lost weight - you only have two to three stone to lose this time...
I know how easy it can be... I maintained a 12 stone weight loss quite nicely until my husband needed 5 operations inside 1 year and I gained over 3 stone... That is why I am back here...
My old friend Cate has a lovely thread on motivational sayings which you may enjoy reading...
http://weight-loss.fitness.com/threads/33533-Motivational-sayings-and-or-affirmations
They may help...
I certainly thought on my original project that the motivation to do great things really can come from within... Maybe thinking about how good you felt when you hit goal weight last time round... (I know that I was in tears with joy when I did it)... I have known people on this forum get major motivation from the idea that the ex would get food for thought when they saw what they were missing and would never be theirs again...
There is little point in me telling you to start being more active today by doing extra walking or joining a gym (whatever you feel up to) and maybe determining to cut out junk... After losing 6 stone you know all that already and have probably done that... If you havent got round to doing it yet - you know that they are the best first steps for stopping the rot...
Since you give your weight as 15-16 stone - I am guessing that you havent been on the scales... Take your courage in both hands and get on there... That way you will know where you are coming from and can celebrate as the weight comes off.
I suggest that you read the following thread - it may help you settle in and make friends here
http://weight-loss.fitness.com/threads/57955-My-advice-to-newcomers
Good luck with your project.