Hey, Alright, after reading this one through several times, I at least know for a fact that this isn't just a question of "stop or continue" or "right or wrong". Reading all that, it would be unfair of me to sit here and say "well what you're doing is wrong so you should just stop". At the same time, I can't condone what you're doing.
Before I start sounding like a politician, I want to say this. I can understand in a sense where this is coming from. You are losing a father figure in your life, your presently without someone to care for you physically and emotionally on that level, you're somewhat used to being with someone in that age range, and most importantly, you're acting this way out of passion. Any bystander to this situation could sit around and tell you that you're a homewrecker or a bad person, but the truth is that it's never black and white like that. Human nature is such that there are never just two sides to every coin.
Your intentions are there. It's not as if you're attempting to split up a happy couple in an act of vengeance or something. You're working towards personal happiness in life, with someone else...and in another context...no one would even blink at that. Here's the problem: Regardless of your intentions, or your motives...I still can't convince myself that the result of this is going to be good for anyone involved, and thus I can't morally say that I think this is the right thing to be doing.
Think about it. What happens if you stay with David? If you continue going about with him as you are? For him, this is ultimately going to end his relationship with Cathy or you. Cheating never lasts forever, something has to give. Whether he decides to leave Cathy or leave you...someone is going to end up hurt, probably both. On your end, your risking a support system that you've had in place for years and years. You're risking fracturing a relationship you've had with both Cathy and her son. You said yourself that this kind of love and care isn't something you can't afford to lose right now.
Most importantly, look at what this has done to yourself? Yes, you're happier when you're with him like that. But it's like a drug in a sense. I think you recognize deep down that this isn't something you should be doing. You're cutting yourself, and ridden with guilt and fear over it. Yet you keep going back to it despite all that.
Look at this for what it is. Look at your future with this. Where are you really expecting things to go. I'm not trying to be mean...but I don't understand how you can come out of this a happier, better person. The things that are tying you to the relationship are related to your own personal desires, rather than your best interests or the best interests of others. That's usually a somewhat destructive path to go down.
I can tell you what I would do if I were you, but the problem is that I'm obviously not. I can't understand in exact detail what you're feeling about this, or what you truly want. It's up to you to decide what you want to do here. Personally, I would draw the line.
I know it's easy to start thinking that once you've had that kind of relationship with someone you can never go back to normal. In a sense, the relationship may always be a bit different. However I don't think that means it has to ruin everything. Tell David that you've had enough. That this is morally wrong, and that the guilt is tearing you apart. Put your foot down...you have to mean it. I don't know what consequence that will have with him, but at least you will be able to look his family in the eye again and have the knowledge that you had the will power to end that.
More importantly, know that there are other ways to go about dealing with this than hurting yourself. I understand the urge to when everything is piling up on top of you. I've been there and given into that. But look back on it. Where is it getting you? It's not solving the problem. It feels good because it's a hell of a lot easier than confronting the real problem...but in the end it's just making it all harder. Don't treat yourself that way. You deserve better than that.
Like I said, only you can really know what you can do here, and it's up to you to decide. I'm just thinking that from the looks of things, this is one of those things in life that is going to end up being more destructive than good. Don't let that spiral downhill even further before getting a handle on it.
If you have more to add to this, or if you ever need to talk to someone, feel free to message me anytime. It would of course be completely confidential.
Take care.
-Isobel