i've joined slimming classes and just cant stick to anything Because you have no confidence, therefore no motivation.
OK, firstly..you need to start feeling better about yourself. You feel you are overweight. In the short term there is nothing that you can do to tackle that, except eat less/exercise more. In the long term you will see results.
Your body has just given life. STOP hating it. No matter who you are, unless you have plastic surgery, you will have reminders that you created life, carried a baby & gave birth to it. Love your body for that, if not for anything else. And if you do, STOP comparing your body to what your dh may see in porn & STOP believing societies ideals of how you should look. Know that celebs don't get trim bodies without lots of hard work (and even more photoshopping). There are unrealistic examples of what real women look like.
If you are repulsed by your body I am not surprised that your dh is giving you space. I don't mean that in a bad way, just that, how can it feel good for him to be intimate with someone who is not enjoying it - for whatever reason! when i do have sex i feel my rolls of fat rolling up his stomach and it makes me feel sick! He will probably be aware of this & is perhaps trying to avoid it all together to prevent you feeling uncomfortable.
Talk to him, tell him how you feel. If he makes reassuring noises about your body, accept them for what they are, BELIEVE them.
I am overweight, and get my confidence from the fact that my dh loves my body. He would love it if I were skinnier too, but it does help me to feel good about myself when he accepts me how I am. I don't even know what my rolls of fat are doing when I have sex (tmi) because my mind is too pre occupied 
I agree that you should initiating sex & seeing what happens. Don't be repulsed by yourself, perhaps 'suggest' (i.e manipulate yourself & him into) a position where you don't feel body conscious & go for it. You may find that once you start back on that track you will start to feel more confidence in yourself, which will in turn make you accept who you are, or give you the boost you need to get the weight off.
Good luck!
Also, it is often touted on here but it is very true,...don't forget who you are. You are a person & an individual & just because you gave birth, doesn't mean that all you are now is a mum. Find time to take care of yourself. Whatever form that comes in, a little shopping for you, a massage, a nice long bath with wine & chocs etc. I find sometimes that even something as simple as lunch out does the trick, just to eat one meal without anyone needing something! And also reconnect on a basic level with your dp. Don't forget that foreplay starts long before any physical act.
I hope you find something that works for you.