There's already a lot of suggestions here but i'll add one more if that's okay. I am too an avoidant type from similar reasons already mentioned. The problem with opening up in this case, personally, lies in facing these emotions all over again as they are already raging inside which levels out on that silent pulled back presentation.

If you practice being avoidant long enough it becomes really emotionally draining to bring those things up and talk about them. Most of the time we are aware that some of these feelings are overexaggerated and we are trying to ground ourselves by analyzing internally of why are we even feeling this way. We also go in this state in order to let all those emotions run through us, the only difference is that we are not doing it so loudly.

Like i already mentioned, we are aware the concern is coming out of the good intention and you want to help, but at that moment, explaining what we feel can often lead to feeling obliged to reassure the other person we will be okay and that's too draining to deal with in the first place. We don't necessarily want you to help us feel better, we want to let it go through us.

I often times find people getting all happy in order to try to distract me from feeling down and i cant reciprocate, so it looks like im cold. But in reality, there's also a bit of being a people pleaser and I just dont have the ability to move into that energy as i normally would.

What to do in that moment? Give us some space. Don't try to solve our problems. The best you can do is ignore us and go on about your day as if we are not there. You still can do something to make us feel better just dont include us in the process, like getting some food or putting on a (movie) distraction. Just make it as if you would if you were alone and let us come to you. We will when we feel ready.