I've heard it said that bitterness is buried anger - and no-one would be more justified in feeling incredibly angry than you. You've had a very rough deal with this horrible man.
I am a great advocate of therapy, having had a lot of it myself. It really is a chance to unpack complex and deep-rooted emotions, with the aim of getting them out for an airing and proceeding through healthy stages of justifiably intense emotions with the support of your therapist. re a therapist gives you permission, if you like, to proceed through uncomfortable emotions, whereas society does not. There is no-one who will sit with you, not even a close friend (though some will try), while you literally live out those stages. A therapist is trained to sit with you, to recognise themes, to guide you along healthy paths. The aim is for you to come out the other side, having accepted what happened to you; having lived out, with support, the deep emotions that didn't get an airing at the appropriate time and have therefore ended up going underground, unresolved. I hope that makes sense.
If money is an issue it is possible to access reasonably-priced therapy - some womens orgs offer therapy at eg £5 a pop (or less!). I wouldn't suggest the type of therapy/counselling that GP surgeries offer - though this could be a start - as I think you need to be in this for longer than 6 weeks (though CBT, which is offered by GP surgeries, could be a good addition to your armoury).
ime I had to cobble together what therapy I could on a limited income. It was worth it - I would be a complete nutjob if I hadn't had that chance to work out, sometimes at a snail's pace, sometimes not, some very painful and difficult things.
I'm sure you know full well that bitterness is incredibly bad for you, otherwise you wouldn't have posted about it. However, you can't always just decide to get rid of it (as I'm sure you have found out): sometimes you have to lance the boil in order for the wound to begin to heal. Society in general doesn't like bitterness and reactes very strongly against it; it is better to sit with someone who is not repulsed by it and whose job it is, after considerable training, to support you through to a resolution.
Try BACP (British Association of Counsellors and Psychotherapists), who have a directory of therapists you can search for in your area. Have a look at their website here, particularly tips on what to expect and how to choose a therapist that is right for you. Once you have drawn up a list, give them a call to make an appt (therapists often offer a free first session) so you can get a feel for the therapy and also the therapist - if you don't click then move on to the next one on your list. Ask them about discounted rates - many offer a sliding scale of fees. Therapists expect difficult q's btw, don't feel you will offend them - they are trained to not take offence and to not have an agenda! I have also been known to not make an appt with a therapist because I didn't like the sound of their voice - it's my choice, I will be working on a deep level with them and I need to feel comfortable and safe.
Good luck OP. I'm so sorry you have been mashed up by life - you're not alone, not by a long chalk..
(sorry for long post btw)