Did I make a mistake, by starting to love.

I had this like girlfriend for 5 years.
She wasnt really a girlfriend, cause she only came a few days a week for 20 minutes or so for s.. and then everysingle time asked for money afterwards.

Later, well, lots of times she accussed me of being a user and that I was just using her for sex. We had huge fights.

But, ok, was using her for s.. sometimes, not all the time, and I did have feelings for her, but I played it, that I will never give my full heart to her in fear of it breaking.

We broke up eventually, then 2 months later, I saw her, and we walked back to my place.

she started talking about marriage again, I said, I cant marry you, cause you cant love.
Anyway that night, she stayed for the first time, about 5 hours.
we listened to music, watched video music, and made love.
Only this time it wasnt the same. The s.. wasnt so good for me, cause the lust was completely gone, and I was having deep feelings for her, as, I took it this time, that maybe she really wanted marriage.
It wasnt in my head, or hers, well, so I thought, that we are just going to have good s.. and we both wanted it.

but, since that night, she doesnt want to come here, she keep saying she is doing voluntary work.
when I go to her house, they say she went to the clinic or something.
but, I just know, she is covering her tracks.

that very next night, at 2AM, I saw a golf with dark windows up the road near the bushes.
I just happened to go outside, cause I was restless and needed fresh air.

The car must have seen me, as it started up, thats when I saw it.

I have a terrible feeling she was in that car, after partying.
the white car, came here once with 2 others to buy my car.
but, I declined on a very low offer.
she was with them that day, and brought these blokes to look at the car.

Is my girlfriend cheating on me, and is it because our lust, like of each other, and s.. part for 5 years, suddenly I gave my heart, which she wanted and promised not to sleep with other girls again.

Was that the mistake. Should we have just stayed s.. buddies.