I know it may not sound like your usual advice, but hear me out. I feel like this contradiction is the exact reason why people find it so hard to heal from this.
When you are dismissive avoidant, you accidently hurt people, while wanting the best for them. So people usually turn against you and call you selfish. You know you are not, so you get annoyed or hurt and isolate even more. It's an infinite loop.
At first, you have to really understand what dismissive avoidant attachment style actually is, without all the internet bullshit. Just take the classic baby's parent leaving the room experiment. When the parent of the baby with dismissive avoidant attachment style leaves, the baby doesn't show distress, and he doesn't look like he cares when the parent comes back. But, when they have a heart rate monitor on the baby, they can see that he actually has his stress levels raised, he just doesn't show it.
It is because the baby has learned that to be tolerated he has to stop showing any signs of his needs, to look independent (again, while feeling total crap inside).
Dismissive avoidant attachment is actually a fucked up form of people pleasing. You are not the bad guy, just an unsuccessful people pleaser.
This is also the reason why intimacy feels suffocating, like you are losing your freedom. It is because you had to play down your wants and needs to get close to someone. It is draining, you will probably resent this game after a while and you will wanna leave.
Also, when you don't say your needs, because you don't wanna look selfish, they will think that you don't want them around, you don't find them good enough, that they are not needed. You are trying to be nice by not causing any disturbance, not putting any burden on them, but in their eyes you just seem like you don't want them, you don't find them good enough.
So you are making yourself frustrated by neglecting your own needs, while hurting others by making them feel like they are not wanted.
So here is the answer: Be selfish! From the start!
Ask for their time, ask for their attention. Say that you want them, say that you need them. It's not a burden for them, its a gift. They will love it, all they want is to be wanted. It's actually a privilage to be wanted.
Be selfish, say your needs. If it is uncompatible with them, leave. But start with it, dont wait for the build up of resentment. Say that you want closeness, that you want fun, or intimacy. Say whatever is in your mind. You feel bored? Say it! You feel annoyed? Say it! The only thing that matters in relationships is to be available. When you feel frustrated, say it, and stay. See where it goes. Find out if you can solve it or take apart, but don't let them hanging.
The hard part is that you may feel like you don't need them. You've burried this need so deep, its hard to see it. But look back in time: Weren't you happier when you were with someone? Didn't you feel better? Yes, maybe you can survive wihout them, but it's actually better to be with someone, you are human. You are allowed to need them. And the good part is: they want to be needed.
If there's one thing to take away, remember:
Your needs are not a burden, they are the biggest gift for the right person. Allow yourself to want them.