@Ks1994 do as much as you can with the NHS regardless of the fact you won't get IVF. There are plenty of steps before IVF depending on your situation. 3 day periods do not immediately indicate a hormonal problem. I have light 2/3 day periods - I have textbook bloods. It was never mentioned as a concern in the slightest when going towards fertility treatment (which in the end we haven't needed).
To get a referral you need:
Day 3 bloods (this tells you so much)
Day 21 bloods
Transvaginal ultrasound
A sperm analysis
Privately, you could have your AMH checked which is your ovarian reserve although at 28 it should be okay. In London you can get it done for around £100. The NHS do not offer this.
I don't think your periods are wildly irregular as long as you're having regular sex but you do need to get to the bottom of this. Luckily, It's not as though they are going 90 days so it super hard to to pin point ovulation, so make sure you're having plenty of sex. You should 100% be having a repeat progesterone test. as long as it's between 6-8 days before your period begins, you should get an accurate result. Was your last one between 6-8 days before your period?
Doctors sometimes say to ditch OPKs because people get hung up on them and only have sex when they get a positive but bodies are funny and sometimes there is a delay in the release of the egg. Normal OPKs can detect hormonal change, but don't confirm an egg has been released, hence lots of people temp - if done properly, this is accurate but depends on your work / home life situation. Do you get EWCM?
Your main concern is you, but it is just as likely to be your partner so please switch mindset on that. He could have developed a varicocele, it could have been lucky with your first child, he could have an infection... there are so many things. As women, we often think it's us, but statistically it is just as likely to be him.