Oh hun you do have a lot on your mind don't you?

Firstly, WRT your depression & anxiety & meds - you are not a bad mum for needing meds. I have in the past (many times) and more than likely will in the future, so have many other mums. It might not be ideal, and I try to avoid it if I can but experience has taught me that if you need 'em, you need 'em - and by not seeking treatment or doing what you need to for YOU, you are not being fair to you or to your family, KWIM?
So maybe you wouldn't need them, or maybe you could talk to your doctor about something that is safe to take while PG & BFing. Also as an aside have you tried counselling? It might help a bit to have a bit of support.
And please, please do not feel bad. I went through a stage where I was a bit 'woe is me, I can't cope without meds, ahhh this is the rest of my life!' and over time I have turned it around and gone well, if we have a physical ailment that we need meds for it's ok - for me, this is the same. I think that some people just need a reset every now & again, at least that 's how it is for me.
Hope that makes sense.

secondly, I totally understand what you mean about the excitement of being PG & having a new baby. And that's normal I think, but it is worth asking if that's the biggest reason for wanting anopther. Do you have a gap to fill in your life? Would something else to look forward to help fill it, or does it have to be a baby? They are questions I have asked myself before too (and FWIW I have come up with different answers at different times).

As for money, well, you manage - but I can't say it doesn't make it a bit harder.

Age gaps - although I desperately wanted a fourth I had agreed with DH that three was enough. When she came along anyway there was a 7 year gap between her & my youngest (and 14 years between her & my oldest!) - and you know what, it's an awesome age gap, not one I would have chosen but it's brilliant, they all adore her, there's no jealousy & they all play with her, yadda yadda.
It's not always a bad thing. Take your time & figure out what you want.
Trust your DH when he says he won't resent you. You can only take what your he says at face value.

You have such a lot to sort through in your mind & I wish you the best with it.