A blunt shove is easy. For starters, consider the following risks due to obesity:
Diabetes Type 2
Bowel and Ovarian Cancer
Fatty liver disease
Anesthetic-related medical complications (eg simply never waking up from routine surgery)
Lower quality of life
However, practical advice is harder since everyone is motivated in different ways.
I suspect you'll have to first discover and then overcome the reasons why you can't stay on a weight-loss programme. It could be lack of motivation, fear of change, boredom induced by a particular diet, bad eating habits or unsupportive people around you. Figure out those and your task might become easier.
Losing weight without the benefit of exercise is achievable - it just requires food energy restriction ("calorie counting"). That's how I lost over 50 kilos. Work out your Daily Energy Requirements (DER), then work out how much less than that you need to eat in order to "burn" fat stores, bearing in mind that your DER will go down as you lose weight.
Explore the articles on the forum's main website to learn. Participate in the forum to help stay focused. Keep a food and weight diary. Report to us! Get encouragement and a gentle shove from people who are or who have been through what you're going through.