Separation dilemma - when to tell DS

[Onlylivetwice]

Long time lurker here which has helped me realise I don't have to stay put in an unhappy marriage.
Problems in marriage have made me resentful and as a result I have pulled away from intimacy.
DH is a good man in many ways - hard worker, does fair share around house. However, his glass is half empty and mine is half full. He is negative and can moan about things that I never would. Flares up at DS over the smallest things. Self-employed so constantly concerned about money coming in and always berates customers suspecting they won't pay. Can get angry quickly over small things. Prone to getting down but won't seek help.
I am past trying to make his world perfect. I work hard full-time with a long commute. I pay half of all bills and earn half what DH does. He rolls his eyes and creates an atmosphere when I get in late due to eg an accident on the motorway. I try to wind down in the evenings after a long and stressful day - he puts the news on and moans about world politics, the newsreader's interview technique or (if female) her appearance.
3 years ago I saw an inappropriate text to a female customer on his phone. I wasn't checking - it just pinged up on the phone in front of me. (He is not techy at all so wouldn't know how to hide texts
from front screen..) I asked him why his customer was sending this winky face 'good to 'see' you again' text and he, of course, downplayed it all. In subsequent conversations he suggested he might be seeking attention as I was too busy (studying for 2nd degree whilst working flat out at the time) !
I chose to move on and accept it and he ceased all communication with said customer which I have no reason to doubt , but it has had its repercussions.
These examples may sound lame but the feelings I had for him have gone. Either eroded or switched off, but I can't seem to get them back.
I have not wanted sex with him for over 2 years as I find all of this negative attitude and 'poor me' attitude such a turn off. This has obviously caused issues and many conversations re our marriage. We have been 'trying' to make things better- he being more positive and me more attentive, but inside I know I want to end the marriage and be in my own. I have a fantastic network of friends, a supportive family and I know my life could be so much better on my own and I also believe he would be happier with someone more suited - someone who would adore and worship him more than I do perhaps. I have held back from actually saying what I really want due to DS (only child) studying- last year of A'levels.
So, here's the issue (apologies I am trying to be succinct!) DS finishes A'levels in June. Flies the nest in September - off travelling for 6 months. I want to come clean to DH re my honest feelings for him and our future together. I just don't know how best to do this for DS. If we agree to separate before he goes away how will it impact on him travelling? I have travelled extensively myself and I know things at home can be magnified when you are away. I just want him to go away and have a fantastic well-earned time after all his hard work.
Any advice welcome and thank you for reading this far!

[ocelot7]

Yr DS will have discerned how things are between his parents & is hopefully old enough to cope.
Maybe tell him in July so he has time to chill after exams plus absorb the situatuon/see how things will be fine & ask Qs before he sets off.

[Onlylivetwice]

Thanks Ocelot- good to hear that perspective. Default setting as parent is to protect them isn't it? DS is a mature and sensible young man. I am sure he won't be surprised.