How do you forgive?

You don’t have to forgive them, I haven’t forgiven my deceased MIL or my DH, what I have done is accepted that she was a troubled bitter old woman and he was a product of his environment. I did the burn letter thing and that helped and as time passed it got easier. For me accepting I couldn’t change the past was crucial but I was and am determined that she will never steal another happy memory or experience from me (even from beyond the grave).
For me, realising that my anger was a healthy form of emotion that I don’t have to repress to keep the peace any more was hugely helpful - when I gave myself permission to be angry it seemed not to sit with me for as long. I did see a counsellor and have done some CBT over the years all of which help stop me in my tracks and think through the emotion to reboot my brain, might that be helpful?
With DH, his commitment to making sure our own DC aren’t subjected to or experiencing the same nonsense the rest of his family seem to thrive on (manipulation, silent treatment, drama etc etc) and his willingness to look at his own reactions has helped massively and he’s a different man since she’s died. I do still sometimes resent all the times he placated his mum to the detriment of our family, but that’s when I use my breathing and reboot tricks so that I don’t perpetuate my negative thoughts.

I agree with what armadillo wrote. My ILs are no longer an issue in my life but there are feelings still there regarding my husband. Not because of a lack of forgiveness or anything but because I haven't seen any evidence to give me the confidence to know he would now have my back. Maybe it's more a lack of trust? Can I trust him to be capable of being strong for me? Of being able to advocate for me? Honestly, I don't have that confidence. I think the only thing that would heal that is being in the presence of MIL and seeing DH be different to the man who removed himself well away from any situation where he would have to deal with MIL, left me to do it and be the 'bad guy' and the meat in the sandwich. All to protect himself from Mummy's wrath. That wound needs DH to prove himself and win my trust back and I just can't see that is going to happen. Not that he's not a fantastic husband overall but there is that thing there.

Am I remembering properly that you lived quite near your inlaws?
Can you move?

Honestly I’d look into couples therapy. That way you’ll have a safe space to learn to move forward.

Your feelings are very valid

I think you may never forget those firsts were taken away from you and are gone forever - the resentment may always be there

What is the plan for your husband to make up for it? An apology may not be enough - too little too late

I've been thinking a lot about this and someone said to me forgiveness needs to be earned.
I don't think that means apologising or making amends, I think it means changing.
Is your husband not making you happy now and that's why you can't let go or maybe something else is making you unhappy and you are looking for a reason?
Usually to forgive people need to trust that they are safe and supported now I think.