If I am making the right sense of your post - then I think there is a transitional period during which you will feel very very lonely.
That is not unusual - as a lot of the posts on this site confirm.
Some people use a transitional relationship (i.e. someone at their side in the hope that they will avoid the lonely feelings) but that can lead to a very messy situation as well. It can confuse the inner journey with an outer reality that has been artificially fabricated and so life becomes even more complicated.
I have yet to hear of an easy solution to this problem - which is unfortunate as I would then be up with Bill Gates in the wealth ratings. I quite like that thought: being rich because you found a way to make people happy, hmm
But back to reality.
When you say that having someone in your mind, who is not alongside, causes you to feel very very lonely, have you considered the difference between your internal representation (mental construction) of this person and the actual person that treats you so badly in the real world?
I'll attempt an explanation.
Falling in love
as opposed to growing to love someone
seems often to happen within a split second - it's that love at first site
My own way of making sense of this is to think that we have a 'template' stored away in the back of our minds that we use subconsciously to scan the people that we come into contact with during our life times.
Imagine a pair of spectacles that have a very fine etching on the lenses that remains out of focus for the purpose of ordinary vision. But when they focus on an image that fits that specially etched outline: wooooshhh!!! you have fallen. People often use metaphors like 'we just clicked' ... 'I fell for him/her' ... and it's not unusual to hear: "I felt I had known him/her all my life!" ... and:"it just felt like we were made for each other!".
So now the flame has been lit, and the internal representation really starts to glow. Usually the burner runs at maximum temperature for the first six to 24 months and then many people become aware of a cooling off period. That's not necessarily all bad, because they may have grown to love the person they fell for, and a lowering of the temperature may actually feel more comfortable - things might feel safer and more settled and "real".
"Real" is GOOD ... in fact VERY VERY GOOD. But some people find that they need that heat back and so go off looking for new models to fit their etching. Maybe based on their previous experience, they think they can modify things a little.
Other people who are content with relationship will look for the thermostat and find ways to turn up the heat now and then - so they get the best of both worlds. A mature sense of reality and at least an 'O' level in emotional intelligence does help a lot in developing and maintaining an intimate relationship over the longer term. But it does take two of each!
I'll stop there with my antithesis of Mills and Boon, because I hope I have explained enough to allow some sense to be made of the point I am attempting to make?
I think you have fallen in love with your internal representation so much that at the moment you are unable to update your internal database, and so although you realise this guy is treating you badly, you just carry on feeling
that there is something right about the relationship.
If you leave this relationship, you will take your internal representation with you. All you will leave behind is the screen you projected your love and caring feelings on to. All the qualities that you used to build that representational image will go with you.
Yes, you may feel lonely for a time because you have no screen, but your template will still be in place and next time that snap-shot moment may capture an image that is worthy of your love.
I hope this makes some sense - I realise it may not.
If it does, I hope it will help you make sense of what is happening to you right now.