Complicated advice needed!

Hi, I don't usually do forums but I'm struggling to decide what to do. Excuse the long story... 
I am married with 2 step children and my husbands ex wife lives near us with the kids. So I feel quite smothered sometimes. I have a very stressful job and have struggled with depression for about 5 years due to taking on a lot of baggage with the step kids. When we first got together I fell pregnant but we didn't feel it would be fair on the kids or his ex if I had the baby so I had a termination. His ex then fell herself. This lead to me taking fluoxetine to treat my depression. I was doing very well and I came off my tablets in July to TTC. We new it was going to be tough but this week my husbands friend has asked him to be best man for his wedding. This is for a friend who is friends with his ex so she will be there along with all her friends. It has sent me in to total relapse and I feel I need to go back on them to cope. I have said I'm not going but I feel sick at the thought of him going. Our lives are so controlled by her and I'm gutted that I may have to prospone my plans to treat this.
If anyone can advise I would be so grateful. I would be taking 20mg fluoxetine. Happy to expand on more detail of interested xxx