diminishing supply?

My lo still has 2 bfs a day at 12.5 months. im a shift worker so once or twice a week LO has had to have ebm in a bottle and ive pumped so my body still gets the stimulation. As of the last couple of months, the amount i get off when i pump has diminished a lot - now 1-2 oz where it used to be 4oz. I froze a lot of milk in preparation for my return to work which has meant when the amounts went down, i was able to draw on that supply and LO didnt need formula before he was one.

Last week the final portion of frozen milk was used and i obviously can't replenish it now, so this morning (as i was on a night shift) LO had 2oz breast milk and 4oz cows milk. He didnt have a problem with that at all, but i feel a bit sad its silly as have done so well but feels like the beginning of the end and im not ready yet.

Ive pumped off another 2oz so LO can have that with cows milk at the next feed i miss. I will still bf him twice a day most days. My OH's mum is coming at the weekend and we thought she could try and put LO down, as my sister is about to have a baby in the next few weeks and would love to visit her so it would help if we could leave LO overnight (never done this before, except for night shifts with his dad).

Im just getting so nervous this will be the beginning of the end, but mostly he will still have his breast feeds just 1-2 feeds a week by bottle which will need to be some cows milk. Will my supply survive after all this time feeding? I just want it to be LO's choice when he's had enough (i dont think i will ever have had enough, but when he's happy i wont have much choice). x