Hmmm; I'm not sure exactly what I can say to motivate you, other than to focus on your underlying goals.
I go through phases of wanting to ride alone and wanting to ride in a group. Right now, I'm in an "alone" phase--at least during the week. It's just working better with my schedule. I've also not been riding particularly well, so it's better for me right now to just go my own pace. There are some routes more than others that I like to ride alone. They have less traffic and are more scenic. There're not TOO isolated, however. So, for me, I feel much more motivated when I'm doing a route that I enjoy.
For me, I really have to create a certain mindset with workouts in general--gym, yoga studio or cycling--that I just will not give myself permission to skip them except when I'm sick or have another commitment. The minute I open the door to letting myself off the hook for any other reason, I backslide into inactivity. I have to admit that I haven't been very good this year about staying in that mindset, so it's not easy, but perhaps that's the approach you need to take. Tell yourself that you have to ride on a given day, alone or not, and stick to it. Hopefully, you'll rediscover the joys of riding alone, and it will eventually become less of a chore.
I'm lucky in that I do have any number of cycling buddies to ride with if I so choose. Could you possibly work on finding new riding buddies who might be able to ride with you in addition to the couple you already know? Are there any weekday/evening rides offered by your club and/or local shops?
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher