purplemonster i know what you mean about feeling like you have nothing interesting to say to your OH in the evenings...I think 3-4months is a difficult time cos the novelty of having a baby has worn off a bit but the relentless hard work hasn't gotten any easier.
It was around this stage that I felt like I had really lost myself somewhere along the line (I also had PND so probably suffered worst than most, but I think the general principal is the same....you need to remember that you're a person as well as a mummy!). What I found really helped me was forcing myself to spend half an hour every day doing something that I enjoyed, and which I would have done Before Baby.
For me, I work in media relations and am a bit of a news-hound. Used to read all the national papers every morning at work. Obviously, there's not a lot of time for sitting around reading the papers as a new mum, but around 4mo I made a special effort every morning when DS was awake and happy to put him in the baby bjorn and walk to the corner shop at the end of my road and buy a copy of the "i" newspaper from the Indepdent which costs 20p. DS enjoyed the walk and seeing the shop assistant at eye level, and then I VOWED to myself that I would find 20 mins to flick through it later in the day: either whilst DS napped on me, or if I managed to get him down in his cot, or maybe if we went for a walk in the afternoon and he fell asleep in the buggy, I would sit on a park bench and whip it out for 10 mins before he inevitably woke up.
This made me feel a bit more connected to the world, rather than just being trapped in 'mummy land', and then it meant I sometimes had something to say to DH in the evening which was not about the colour of DS's poo, or the length of his naps etc etc. Even just being able to say "Hey, did you see that story this morning about Kim Jong Un/that skateboarding dog, that footballer's record transfe" made me feel a bit more human. If current affairs is not your thing you could do something different: a set-in-stone weekly walk to the library to choose a book for yourself, and vowing to read 10 pages per day? Sky Plussing a drama series and watching one episode per day whilst LO naps on you? ANything to have some escapism from being mummy.