Why should he marry you - he clearly does not love you. Actions speak louder than words - just look at his actions, and see if they are they actions of someone who cares about you.
Effectively, you have three choices - accept it as it is, try to change it, or end the relationship.
I think you need to give yourself breathing space, and talk to a counselor about this problem, so that you can clarify your options.
Basically, how do you feel about living with this for the rest of your life?
What are the prospects of him changing?
fact is that until he accepts that he has a problem, and will accept help for it, there is nothing you can do directly in that area.
However, you need some support yourself, and to find a way to make your life easier.
I have several suggestions for you.
First off, yourpartner needs to be confronted with your feelings about his behavior, and made to understand that, while you care for him his behaviour is unacceptable and will not be tolerated, at least by you.
He also needs to understand that that any continuation of his lying and cheating will have consequences. They need to be spelled out to him very clearly, with clear emphasis on the fact that they will apply immediately. Cheating once more, or continuing to lie means it's finished.
We humans only indulge in behaviour that brings reward of some kind. Only when that reward (whatever it might be) disappears, or the consequences of our behaviour promise to be unpleasant do we consider changing what we do. Therefore, you need to give him a reason to change, otherwise he simply will not do do. Why should he? He has gotten away with his behaviour until now, so it works for him.
Here is the clue to sorting things out. When you are faced with non-co-operation - give him choices, and make sure they understand the consequences of his choice - and always follow through. If you don't he will continue to take treat you the way he is doing just now.
Ask him too, what he is prepared to do to change his behaviour in future - tell him to research what might help him, what professional help he might get, and even consider a ‘contract' between you. In other words, involve him in his own change, with a prospect of a small reward for success and dire consequences for failure.
However, don't get angry, stay cool and in control, matter of fact and stick to the facts. Avoid drama. I know you are frightened, but do you want the rest of your life to be like this?
I'm going to suggest that you would benefit greatly from a course of Cognitive Behavioral Therapy. It is a form of therapy that addresses problems in a direct and targeted way and is brief compared with most other therapies, and would really help you think more clearly about your situation and help with your fears and anxieties
CBT is based on the fact that what we think in any given situation generates beliefs about, and reactions to that situation, and also cause the behaviour and feelings which flow from those beliefs and reactions.
These ‘automatic thoughts' are so fast that generally, we are unaware that we have even had them. We call them ANTS (automatic negative thoughts) for short.
If the pattern of thinking we use, or our beliefs about our situation are even slightly distorted,
the resulting emotions and actions that flow from them can be extremely negative and unhelpful. The object of CBT is to identify these ‘automatic thoughts' then to re-adjust our thoughts and beliefs so that they are entirely realistic and correspond to the realities of our lives, and that therefore, the resulting emotions, feelings and actions we have will be more useful and helpful.
Cognitive therapists do not usually interpret or seek for unconscious motivations but bring cognitions and beliefs into the current focus of attention and through guided discovery encourage clients to gently re-evaluate their thinking.
Therapy is not seen as something "done to" the client. CBT is not about trying to prove a client wrong and the therapist right, or getting into unhelpful debates. Through collaboration, questioning and re-evaluating their views, clients come to see for themselves that there are alternatives and that they can change.
Clients try things out in between therapy sessions, putting what has been learned into practice, learning how therapy translates into real life improvement.
Please visit this website for much more detailed information on CBT: http://www.rcpsych.ac.uk/mentalhealthinfoforall/treatments/cbt.aspx