I've never posted on here before, but I really feel for you, so here goes...
Whilst I have never been in your situation, many years ago I became pregnant (despite contraception) in a fairly new relationship - although not as new as yours. I had an abortion without telling my boyfriend, as a result of this some of the very few friends I did tell stopped speaking to me for a time as they thought it was wrong. I thought I was doing the right thing as it wasn't 'fair' to upset him or put the burden on him, when it wasn't his 'fault'. Looking back on this, I cringe because I now see that I had so little self esteem, and expectations of what a 'partner' should do, that I thought it was entirely my responsibility to deal with and I buried all my emotions about it. Deep down, I thought if I told him he would leave me.
Fast forward 18 months, when he inevitably dumped me, over the phone, shortly after we had been discussing moving in together, I came so, so close to telling him I had aborted his child. At the time, I felt, similarly to you, that I couldn't move on unless I was truthful with him. I was also (subconsciously) very angry that he hadn't been there to support me - I know that is irrational, as I hadn't given him the opportunity to do so, but there you go. I very nearly told him one night when he cut me off saying 'if this is going to make me more stressed out, then I don't want to know'.
Now, I accept there is probably a lot of projection here, and I know nothing about your ex, but I think you need to consider really carefully what exactly you hope to achieve by telling him. If his reaction was in any way callous, would this make you feel even worse? If he was upset and kind towards you, would you hope to rekindle your intimacy? I think this would be very dangerous for you when you are so upset. You really do need to unburden yourself about this, but I really think you should consider counselling before going to this man, as he may not give you the closure you need. Of course you are desperately upset, and you are absolutely entitled to be, but there may be no easy way out of that upset - you might just have to feel your way through it for a while - rather than thinking you will get relief by telling your ex. For whatever reason, this man has not been there for you: you have coped amazingly so far and you will continue to do so, but please allow yourself to grieve and feel let down: you have been and that hurts.
To finish this monster essay (sorry!), I am now highly relieved I did not tell him. He means nothing to me now. I think not telling him in the first place shows I did not trust his reaction and so why would I have brought that down on myself when he had dumped me? I would only have been doing it to get some form of reaction - and it probably would have left me more bereft and angry than I was to start with.
I really hope you are okay - please do think about talking to a counsellor or therapist about this. Thinking of you.