@Eggandbeans - good to see your most recent post. It can not have been easy to go through the many comments.
As @Heismyopendoor points out, your father could have started a new relationship with someone from, or moved for work, anywhere distant to where you were as a child.
HOW your father managed this move away, how he maintained contact with you, & nurtured both you & your relationship with him underpins much of your current grievance.
For an early teen it’s hard to miss a parent when you cannot act on those feelings by travelling a distance as you need or please without help with travel & money. Particularly if those feelings are skewed or denied due to family loyalties from a broken marriage.
Understandably these feelings can cut deep & will resurface at times, such as now when you are going through IVF & the emotional whirlwind.
Other posters have suggested that you find a counselling professional to help you work through these tangled feelings, particularly as this is affecting your family relationships now & will add to the pressure of the IVF. I strongly support those suggestions.
You have not said if your IVF is NHS or private. I assume that a counselling service will be available either way as the IVF roller-coaster means that people need that extra bit of support. Please ask what is available. There is always Relate who specialise in family relationships, have a sliding fee scale & you can go on your own.
I mentioned a silver lining in my previous post. This situation may mean that you can see your Dad, just the two of you together. I suggest you take this offer up & make it a good an experience as you can. Sooner rather than later.
But before then I suggest that you write to both your father & SM to apologise for any hurt that you may have caused. The Granny business is moot. It is the right thing to do. A brief letter because you can be clear & neutral, so no risk of your emotions clouding the message, & because that message stands no matter how it is gone over. Get it done & dusted as soon as possible.
Turn this round by putting the worst of this right & then take up the opportunity to spend time with your father.
‘Dad, I realised that I really miss spending time with you, when I come down I’d like us to go out, just the two of us, for the day & have lunch’