I will start off by saying I do not know the details of your mom and step moms split and the details truly do not matter. I will say being a lesbian myself I know all relationships are difficult but gay relationships come with a whole other set of difficulties. I do not know where you live but even the most liberal of places have prejudices and this makes it difficult to have a relationship if you are having to hide your relationships especially if one party is ďoutĒ and the other isnít. Believe me I have personally dealt with this, Not to mention I think maybe your mom should have spent some time dealing with her and your dadís relationship failures before jumping in to another relationship so quickly.
Now I am the step parent to a wonderful 17rd old boy his mother and I have been together for almost 6 years and have battled many issues in our relationship which has not made it easy but we did not give up even when times got ugly. I personally do not see anything wrong with maintaining a relationship with the step family. If your mom has an issue with it the issue is hers alone. Do not feel bad for the choices you make. She is an adult as you are and both of you are free to make your own choices as to who you maintain a relationship with. It sounds like you are a very smart, tough, level headed woman despite the challenges you have faced so kudos to you.
Never feel ashamed of who you are or the fact that sometimes you may need help coping with life issues. There is medication on the market for a reason. I too have struggled with addiction so again I can relate and admitting you had a problem is great. You can overcome the depression I promise. Prayer is a good choice in that. Also meditation and breathing exercises will help. Just try to stay positive and tell yourself you can do it. It may take weeks, months or even years but you CAN DO IT! I am glad you have found a good relationship and like you said if you try to work thru your problems and just donít give up I think you will have a great life together. If you need an ear I am here. Like I said I have been in the same situation just a different role.