Pregnant, Relationship stressful and Im falling apart..help?

Hi I dont know what to do, Im falling to pieces. I am 15wks pregnant, we didnt want to be pregnant really until another year or so after we had lived together and worked out if we could live in a blended family together as my partner has two children from a previous marriage. He has them 5 days a fortnight. After much stress and angst we decided to trial living together and see if we could bring it all together, well all that has happened is a million issues have surfaced, children, finances, right down to little things...I am so stressed out and cannot even see straight and it all culminated this morning to me chucking a rage and practically going crazy and ripping my xmas tree to bits. I have lost it, the stress and complications are killing me. I do suffer from depression. I have found I dont have much say in many things and my partner acts defensively to any issue ihave i feel like im fighting for an inch. I need to matter to