unusually tricky situation

Hi, i’m quite nervous writing this for fear of what will be thought of me, and i’d just like to say first that i will welcome both criticism and advice.
Myself and my fiance have been trying to conceive since November, and i was recently told it would be difficult for me to ever have kids, i would need medical help which i have not yet received. Basically, my chances were below half.
This is the part that will sound like i’m supposed to be on a chat show! On the 8th of may i was already 2 days late for my period, it had been 30 days since my last one. but i have never in my life been regular, 6, 7 even up 9 weeks would be normal before i went on the pill 3 years ago. having come off that in Nov it’s been 6, then 5, 5, 4 kinda thing. anyway, that day i i ended up sleeping with my ex boyfriend. he was kinda like my first love, myself and fiance had been having difficulties and this guy was also going to be leaving the country in 5 days. Basically i was stupid. i have of course told my fiance who has forgiven me. the thing is, i still haven’t gotten my period so i’m now into the realms of, if i am pregnant, it could be his. I’ve been taking home tests left right and centre but they all say negative. my fiance also knows this, we’ve talked about it at great length and he is ok with whatever the situation would be, we would keep the child, if it is my ex’s then it’s up to him if he wants anything to do with it, but we would raise it together.
Obviously this would put a huge amount of strain on our relationship and only time will tell if it will last. I just never thought i’d be pregnant and not know who the father is, seriously, i’m not a slut, i’m actually extremely loyal, it’s just this guy was my first love and well…no excuses really. it all just seems totally surreal.
Also, i have all the usual symptoms of my period, cramps, lower back pain, bad skin, moody, bad hair etc etc. unfortunately they’re also the symptoms of beng pregnant!
It’s the not knowing that is driving me insane, i’m finding it very hard to deal with it, i’m also depressed, have been on anti-depressants for 3 years, and have also tried self-harm. this situation obviously is not helping!
anyway, i just needed to vent a bit i think 🙂 As i said, i would appreciate any comments, both criticisms and otherwise. Thank you.