Hi Natalie, I have only just come across this thread and have read every post on here.
I cannot add further to the excellent constructive advice, it is thorough and really makes sense. I just want to add a few comments, if I may.
Sometimes the "noise" gets in the way and it blocks the situation, preventing any hope existing that there is light at the end of the tunnel. By noise, I mean the confusion, hurt and frustration. Feelings become increasingly negative, its a vicious negative cycle. It is missing the point to judge you as a "bad person", and i think that has been recognised by other posters. I sense that you venting on here is probably the first release valve you have had to the pressure cooker of your life. It doesnt make you good or bad, it sounds like you are just finding it soooo tough knowing how to deal with and relate to a couple of teenager who sound remarkably normal, to be honest. You have recognised they are young, naive and dont know everything. I have a funny Simon Drew card which says something along the lines of
"Advice to Teenagers: are you tired of being harrassed by your stupid parents? Act now! Move out and pay your own bills.... While you still know everything!!"
My belief about making step parenting work tends to have two themes that crop up very often - its about the step parent having the backing and support of the partner to set reasonable disciplinary boundaries, and about feeling a sense o f control, in a positive healthy way, that you have choices about the situation.
Firstly, it is critical that the Step parent has the emotional backing of the parent, and visibly, to the children. This should manifest itself, not by ganging up on the children, but ensuring where discipline is needed, both parent and step parent speak with a united voice about what needs to happen. Young people need, in fact want loving boundaries, it shown they are Cared about. It cannot happen if the SP does it and the parent is lax, indulgent and trest their children like "their mates" aka Disney parenting. It doesnt guarantee that the Children will automatically love the step parent back but it keeps those healthy boundaries, keeps order in the home and a bonus might just be that respect will be built. Big caveat here... It takes a lot of time patience and motivation of both parents to do this, working together. If the step parent doesnt feel able or willing to invest emotionally, they are unlikely to be giving it a fair chance to improve. My assumption is that Maybe you are currently too angry, and feel hate, rather than being able to allow yourself any positive emotions (agreed, you may have good reasons, it is your choice) you just want to distance yourself - this is not a judgement just an observation. If you felt that some counselling is appropriate, it could help you to remove the "noise" the unhelpful emotions and start to create a strategy with your husband to regain control.
Which brings me to the matter of control, another key aspect. In this context I mean, that you have some sense of regaining control that you lost when your DHs children came to live with you due to the adverse circumstances of the children's mother. I agree your words used to describe mental health are not appropriate, but taken within the context of your situation, your anger and frustration have been manifested with those scathing words. Someone i dont think you have met or had any relationship with has effectively put a bomb under you life and it is you who is picking up the pieces.
I think that for your own sanity, it would help if you can take the two above themes (united front with your husband + regaining control ) along as a starting point for counselling. Maybe now you have got to the end of your tether, its time to reset the clock and wipe the slate clean and take small steps towards a more positive future. Focus on a few achieveable goals so you at least get some respect from your DSCs if nothing else. You are the adult, in fairness they are still children in terms of emotional maturity!
Only you can decide how much motivation you have, but if you have come to Mumsnet I assume you DO want to try something right?