You're wise to be worried that this is a tipping point, OP. I think it is. You've summed it up pretty well yourself. You don't fancy your DH, you fancy other men and you're avoiding sex with DH and regarding it as a chore. Yet you know that you love sex itself and therefore feel achingly depressed that you might never have exciting sex again.
Logically, you also know that another man isn't really the answer, especially if sex was fantastic and exciting when you first got together with DH. The rational side of you knows that this too, will pass.
Some things you might find helpful to answer are:
What could your DH do that would make you fancy him more? This can range from the mundance to the extreme e.g. lose weight, take more care of his appearance, make more effort for you etc. The extreme would be to have an affair himself, with someone who thought he was absolutely gorgeous and brilliant in bed. Would any of those things make you re-consider his qualities as a lover? (And if you're shaking your head thinking that no-one could feel this way about him, think twice - oh yes they could.)
What are you resenting about your DH at the moment? Resentment is a stone bonker passion-killer.
What is communication like between you? Are you honest about your feelings, hopes and desires, or are you both "chugging along", pretending to each other that everything is fine - and if you're not arguing, it must be okay, mustn't it?
Having answered these questions honestly, then try to step into DH's shoes.
He gets his self-esteem by being respected and desired. Yet more often than not, he sees contempt, resentment and repulsion in your eyes. Yet he can remember a time when you thought he was wonderful, when you couldn't keep your hands off him. He wants that woman back and remembers how much better he felt about himself then. Although he'd never acknowledge this about himself, he's as vulnerable to an affair as you are.
At this stage, you're probably both remaining faithful to each other because of a combination of morals and lack of opportunity. But those are not good enough safety nets.
The key to this is really good, honest communication. It will take a while and many chats - and lots of really active listening. For myself and all the women I know, opening up in this way and getting matching honesty is the first step to re-awakening desire.
This means telling him the answers to the questions I've posed above and asking him some yourself. Being honest about your crushes and telling him that you're worried about what they mean.
You will have seen on the many affair threads here that everyone bewails the fact that their spouses didn't tell them how vulnerable they were and how they were feeling prior to their affairs. So, this is the gift you'll be giving your DH - and he you.
Happy to help more if you need it.