Hi Tilly and thanks for writing to JA
I am sorry to hear about your situation. That said, I have seen the same exact scenario many times over. As soon as the time approached when you were willing to make a significant change in your status as goes your relationship with your lover, he got cold feet and backed off your agreement. This is quite common indeed especially in long term arrangements such as yours.
You see in some ways your lover has had the best of both worlds. You were there for him for the "fun" parts of an intimate relationship and yet not the "messy" parts that we all commit to when we engage in a more committed relationship. Were this dynamic to change there would be more demands on him to engage with you in matters that are not as easy and enjoyable as that you have shared over the past eight years.
Without knowing you both and knowing much more detail about you, your marriage and your lover and his life it would be impossible for any of us here to advise you in any appropriate way. Since you are considering making such a drastic change in your life have you considered seeing a therapist for some brief treatment to help on this?
Let me know what you think and how I can best help you further on this?