Ok I'm being a hormonal wreck and just a silly girl. But please reassure me it's making my pregnancy hell.

Ile start from the beginning so it makes sense.
I broke up with my partner and had a stupid one night stand on the 3-4th of July I then got back with my boyfriend, we had a period on the 20th of July I can't remember how heavy it was but I took a pregnancy test and it was negative. Me and my partner decided to try for a baby when we had this period so from the 20th of July we tried, we had lots of negative tests and even a scan that said we was not pregnant but luckily that month we fell pregnant! So our midwife dated the pregnancy from the first day of our last period and the dateing scan was exactly the same so I'm 14weeks today ( they add 2 weeks on or something?) so this baby is 1 million percent my partners right? I know I must sound silly as I had a period and negative tests and the dates are right I just worry so much and it's making me sick. Please reassure me x

There is no way to know that this baby is "1 million percent" your partner's baby. To be 99% sure you will have to have a DNA test done after the baby is born.

First, I've never heard of a man having a period. So YOU may have had a period, not "we". Second, you are being totally unclear about the timing here. So its really impossible to tell. You say the midwife estimated 14 weeks, but when did she tell you that.

Finally, do you really think it’s a good idea to have a child with someone you recently broke up with?

You can have a light period and be pregnant.
You can have a negative test result and be pregnant.
The ultrasound is an estimate only.
Dating conception from LMP date (July 20) and then going back 2 weeks is an estimate only.

So yes, the one night stand guy on July 3-4 could be the father.
Trying to conceive starting July 20, after sex with someone else only 2 weeks earlier, is going to mean it's a toss up.
AND, since most women ovulate mid-cycle, if you are really around 14 weeks, that means you conceived around July 6, and if you didn't have sex with your partner til later, the odds of the one nighter increase... but really for now it's 50/50.