SCROLL DOWN FOR SUMMARY / TLDR

You're still hurt about what has happened and the actions that he has taken and the actions that is /HASN'T/ taken to resolve the situation.
	If he asks what the situation is: You said you were open to having a threesome /in the future/ and he took that as the relationship is an open relationship.
	Sexuality is a really important part of the relationship and it's something that both people in the relationship have to 100% disclose and be honest about so shoes aren't stepped on. Before even contacting Sara and getting all horny with her he should have 100% made sure that if this was what you meant because he broke this line of trust that you two hold dear.
	You two have been in a relationship for over a year you would think by now that he would know the do's and don'ts of what he can and can't do sexually with or without you.

Anyway he does things with Sara without even letting you know which is bad for you because he doesn't want you to know from how he's reacting. He's actively avoiding you and all bad conversations which he can't do forever and has to confront them eventually. From the way he is acting you don't trust him and what he's been doing because he isn't /COMMUNICATING/ with you at all and is /ACTIVELY AVOIDING COMMUNICATION/ which is a /REALLY BIG FUCKING RED FLAG/ in a relationship. If you want this relationship to work he can't assume things are ok on your end.

He has been spending more time with Sara than with you and that makes you feel tossed aside, alone, easily replaced, and hurt. Just because you're sick and you have cousins over doesn't mean that he has to go try to stick his dick in something else. There's something in a relationship called /COMMITTMENT/  and both sides of the relationship have to stay /COMMITTED NO MATTER WHAT INCONVIENCES COME UP/ and it's not like Alex does anything else. He doesn't work and just plays videogames all day. He can take time out of playing with friends to talk with you for a little bit. And if you're too busy you can always TEXT AND MESSAGE EACH OTHER TO COMMUNICATE. It doesn't always have to be on skype or discord

If he complains about being stressed out than he should go out of his way and out of his comfort zone and try to resolve the situation instead of avoiding answering questions like the plague and walking away from all confrontation. He has made little to no effort on trying to resolve the issues that you two have. The vibe Alex is giving off has been extremely selfish and completely disregards how you feel.
	Examples of this being "you're stressing /ME/ out" and "you (Grace) need to chill out" like no shit sherlock
If he doesn't know how to apologize he better learn how to and fast. Saying I'm sorry isn't an apology and I learned that the hard way. It's not just saying the words "I'm sorry." It's admitting your mistakes and what you did wrong, how you can resolve the situation, and how he will take steps to ensure that it doesn't happen again"

TL;DR:
He should have made sure the sexual lines in the relationship and what he can or can't do
You're still hurt about what has happened and he doesn't give a shit and/or makes no effort to apologize
It's incredibly hard to trust him now after what he has done and what he IS doing
Complaining about stress doesn't do shit he's 18 with  no job and no bills to play he plays videogames all day
Alex is being selfish and arrogant of the situation, thinking that he can just push everything under the rug and move on which he CAN'T but also REFUSES to admit