Hugs, what a dilemma. Having created and bought a child into the world, and knowing the absolute joy (and yes the frustration, hard work, boredom, lack of sleep etc), I couldn't bring myself to end that potential. I was very pro abortion in my younger years. Now I think I am pro choice, but it is not a choice I could make.
Consider..... that your oldest is growing up fast. He/she may become a great help to you. If you become a stay at home mum, would your loss of earnings be cancelled out by not having to pay childcare? Your hormones will be running amock making your decision even harder.
I imagine (perhaps someone with 3 kids could confirm), that having 3 kids, is not 3 times as hard as having one.
You have gained a wealth of experience with your last two. You could do this with your eyes closed (if not sleeping!). Don't underestimate yourself and sell your self short.
Is there a chance you would forever regret the decision to terminate?
I think you need to have a long hard talk with partner and tell him your doubts are in no small way linked to the state of your relationship. This could be crunch time. If you are looking for an out.... Here it is. If you want it to work... It starts here.
All the best with your difficult decision xx