Welcome to the Forum, it is pretty good move, even if hard to do, as you get to see the experiences of others, and that can help a lot.
I have an ongoing anxiety condition and know that it distorts my thinking at times, raises doubts and feelings I'm unable to complete things. It is sometimes more than hard to distinguish between the facts and a perceived large problem that is actually trivial or does not exist at all. In my case such thinking is bought on by increased stress in my life.
I get round this problem by talking with my partner, who is very familiar with my mental health conditions and can point out things in their true perspective. My psych performs the same function. I actually trust them and respect their views, I'm lucky.
You have just made a huge commitment, part of which is financial. I'd imagine you both have a mortgage and this can seem a huge ongoing burden all by itself.
Perhaps even more importantly you have committed yourself to a life with your partner by buying this unit. Now that's not a bad thing, in fact it lends stability and confidence the relationship will last.
However as you are prone to anxiety it is not surprising that such big, significant and stressful actions will in the shorter term take their toll and raise your anxiety levels to the point you doubt yourself, your love and naturally thinking of past less successful times.
Can I suggest you simply keep on with your medications and if necessary ask your doctor for therapy too? Then see how you go.
I'd also suggest, my apologies if you have not already done so, to talk about your anxiety condition with your partner so he understands the type of thinking you might be prone to. If he wants to know more about anxiety consider taking him along to your doctor to get an explanation or simply looking up the facts here
Please let us know how you go, it is a difficult time but if I can improve to the extent I can realise my thought are unrealistic (though sometimes with help as I said) then I'm sure life can improve for you too