It may help you to approach this thing from a different angle. Rather than seeing losing weight as a diet look at the whole thing as about developing a new healthy lifestyle. Just like your old unhealthy lifestyle didn't just happen overnight, your new lifestyle may take some time to nail down as well. The word diet suggests something that has a beginning, middle and an end...lifestyle suggests forever.
Success is achieved when we get our heads in the right place. Once we have the right mindset; the body will follow. There are three aspects to getting the head in the right space. I call them the ABC of the weight loss mind.A = Accept.
Accept responsibility for your life. The bottom line in each of our lives is we got the way we are because of the choices we made. In your example you pointed the guilt of your boyfriend. His actions may be unhelpful but nobody but yourself decided to partake of these foods in a gluttonous way. Someone may give us matches but if we are the one who started the fire - we are the one responsible. Once we have put the problem's responsibility where it belongs then we can be accountable for it.B = Believe.
Dare to believe. Fill your mind with the possibilities. Think of all the advantages weight loss will bring. Fill your head with those visions of a better future. Remember your past successes (it doesn't matter how small, big or unrelated to weight loss)...if you have succeeded in the past; you can succeed in this future matter. Success breds success. Those who think they can - can...those who think they can't - can't...and both of them are right! If you don't believe in yourself, you are doomed before you start. Fill your head with belief!C = Commit.
You have to count the cost and sell yourself on the enterprise. To commit means change. Change always comes at a price. By committing to weight loss you will be changing your lifestyle and your relationships. Lifestyle change means changing who you are. It is not about doing the right thing - it is being the right person. It's about changing your character...who you are even when no one is looking (accountable to your own self and not only to others). With this character change and the resulting worldview change, relationship dynamics will be altered as it changes in response. At the end of the day your mindset must be as follows: Weight loss is not a wish nor a want - it is a MUST.
Wishing you a success journey towards a better future.