I second those who say that it takes much longer than 4 months to get over a long relationship. Think more in terms of 1-2 years. (I was with my exH for 30 years married for 22. Found out he had cheated repeatedly through our marriage.)
That doesn't mean you will feel the way you feel now for the next two years. It will be an upward trend, but patchily and sometimes it feels like you are going backwards, but overall it will be getting better. By two years down the line it definitely feels like something which is well in the past and you won't be able to believe that you went through so much pain over him.
I'd suggest that you need to start thinking now in terms of building yourself up. If you are going through all this pain and change it might as well be transformative in a good way - yes?
Focus on you, on getting fitter (exercise really helps and try to do some every day even if just a walk - I found that my mind was racing in circles at this stage and something like an hour's step class was the only time my mind shut up and gave me some peace), learning new skills, e.g. languages or work related, going out to talks, lectures, meet up events - anything which takes your fancy and on making new friends. Basically keep yourself busy in a positive way. There will still be sad times but they are lessened when you have other things to think about.
And definitely yes to a counsellor. I am so much not a counsellor type or person, but at the stage you are at now and for a few months afterwards, just having someone to talk to who had no agenda other than my benefit and who I knew would not talk about what I said to anyone else was a lifeline. Over time she suggested things like CBT and breathing exercises which also really helped.
And take control of the divorce process too. Because now you are a grown up who puts yourself first and values yourself. Go and see a solicitor. In a long marriage you should be entitled to half the assets, half the joint pension pot and maintenance for life. You need to be sure of your financial position and you need to get at least to decree nisi for the courts to make a maintenance order. Don't settle for less than you are entitled to in an attempt to be nice. It is a waste of time and you will bitterly regret it in the future.
The really interesting thing I think Mumsnet has shown is that many of us who have been through long marriages and then divorced due to infidelity are now, down the track, happier than we have ever been before and would not go back even if we could. Welcome to what will be the best years of your life!