It is good that you feel the urge to repent however you also seem to be going on the extreme side of it by despairing from the Mercy and Forgiveness of the Almighty. Your suicidal thoughts do not sound Normal. For this you must seek some medical attention and or even counseling.
My Dear sister your first grave sin is to have a relation with a Na-mahram. Hence you should rectify that! The act itself will cover the other sins that follow (like you just mentioned)
There is no Harm into wanting to marry a good man and live a prosperous life after. But everything has its course. If he is sincere about this relation then he should send in a decent proposal, after which you can become Mahram and will have to worry no more regarding such issues.
On a side note, his fast is in place.