I'll try to keep this brief!
I'm married with a beautiful new baby and very much in love with my DH. My former 'best friend' was male. The friendship was purely platonic but it was a brilliant one and really my only close real-life friendship other than the one I have with my husband.
He met a new gf 18 months ago and started letting me down all the time - not just in a 'I'm too busy for you now' kind of way, which I would totally understand, but he would actively arrange things with me and then not show up, or be very distracted such that he might as well not have showed, or rearrange at the last minute. I put up with this behaviour for ages because he was having a bad time mainly with the new gf but also with work (we work for the same company), ex wife, family - and I wanted to be there for him but he became harder and harder work.
When I told him I was pregnant (my first DC) back in March he cooled completely. I don't know why. Our friendship struggled on for a few more months with lots more letdowns. We met up in June and he was distracted but there was nothing to suggest we wouldn't continue to be friends in some capacity. Then I heard nothing more. I decided to finally get the hint. In similar situations in the past I'd always been the one to crack so I decided I wouldn't this time, and see if he did (I know this sounds childish and I was probably overthinking it).
Well he didn't crack and neither did I. I spent a lot of the Summer and the later months of my pregnancy feeling anxious about him, wondering if he was ok and missing his company. As time went on I started to feel angry towards him for the way he had gone about things i.e. phasing me out through a series of let-downs rather than just telling me straight. It seemed cowardly to me, and he knows me well enough to know how upset I would be about it - I don't have many REAL friends so the ones I do have I'm very close to and get very attached to. My birthday came and went and nothing.... I had my baby and nothing.... By this time yesterday I was over it, had grieved and moved on or whatever it is you do in these circumstances, I really didn't think about him that much and had worked out I was probably better off without him.
Well yesterday evening totally out of the blue 5 months after our last contact I received a very long text message from him along the lines of there being a big hole in his life that he can't fill, part of him has died, how much he thinks about me and the things we did together... it was like a message you'd send to a former lover. The general theme was one of regret at letting the friendship go. I don't know what's prompted it or what his motivation is - he says that he knows I must hate him (I don't) and that he doesn't expect me to contact him, but then says he wants to know that I'm ok 
.
So now I don't know what to do. Half of me wants to respond and maybe pick the friendship back up if he's up for it, but perhaps invest less expectation in it. The other half of me finds it hard to forgive how he treated me and thinks I should just leave things be, not respond to the message. When I think about it I do miss him and I'd like to be in touch with him (I'm feeling quite isolated at the moment), but not if it's going to sap what little energy I have as a new mum anyway.
What shall I do? Ignore or respond?