Love&Security making me fat😫

Robyn-Rhianne • ❤️
My boyfriend is honestly the best thing to happen to me, I couldn't ask for anyone better. I know he wouldn't change me, and I know he loves me but by knowing that I've let myself gain too much weight. From a UK 10/12 to a UK 16/18 is heart breaking to me. I now have visible stretch marks on my upper arms, all up my thighs, back of my knees and so many places I don't want to know about. 
I wanna be how I was before I realised how much he loved me, I wanna go back to when I loved my body the most.
So I need tips, like, big ones. My depression is often debilitating and results in me barely leaving my house so the whole exercise thing is difficult. 
What do I cut out? What small things can I do that won't completely change my schedule and routine which I have barely gotten used to. 
Help?