Following on from discussions about difficulty a couple of pages ago, I would recommend anyone struggling as I am/was to make their way to Diamond City sooner rather than later. I was putting this off as I didn't want to advance the main quest too quickly, but was getting in a grind with the BoS/Minuteman quests which give a lot of punishment for little reward and not much scope to sell loot. The crunch came after doing a settler mission and finding myself 50% radiated after dealing with molerats and putting a water purifier in the seriously radioactive puddle (the old drive in movie site). Down to one Radaway and the next mission hit me up with more rads dealing with the ghouls at National Guard. No radaway in site and couldn't find the roaming doctor.
I finally visited Diamond City when I was L36 and I wish I hadn't as I'm being railroaded into the main quest.
An alternative route for people finding the game difficult is to do what I did:
Don't
just carry on with any faction's quests. Not even the minuteman's radiant quests about establishing settlements. It will quickly get more and more difficult even with those as you end up having to manage a dozen marginal settlements.
Instead, explore extensively in the north of the map. With a few exceptions, areas there are significantly easier.
Build and craft as much as you can. You can quickly reach L20 with little more than building and crafting. It might only be 1-15xp per thing, but if you build and craft 100 items (only 1 small building and some food and chems) you can easily get as much XP as you'd get from finishing 3 or 4 quests. Levelling increases your power a lot more than it might seem (especially if you choose your perks wisely) and can enable you to significantly improve your equipment.
A specific piece of advice about the Starlight drive-in site:
Avoid the water until after you have gained control of the settlement (as you had, since you were building on it). Enter building mode
and scrap the barrels of radioactive waste in the pool. This immediately removes all radioactivity from the water.
Edit:
On the subject of settlements, a few things that might make it easier:
Save up specific clothing for settlers doing specific things. So, for example, all my shopkeepers wear suits and all my provisioners wear combat armour. This saves you accidentally reassigning a settler to a different task when you intended to assign a settler to it who wasn't already doing something else. Note that any unnamed settler assigned to be a provisioner will be renamed "Provisioner" and will keep that name even if you assign them to something else.
Despite what the ingame tip tells you, any settlers not assigned to a task will default to farming if any plants aren't being attended to. Not scavenging. I haven't seen any evidence of any settler scavenging, although it would be easy to miss a few extra junk items being added to the workbench inventory that already has hundreds of them.
Build up one settlement before going on to another, where that's possible. The most convenient security is turrets - they require no settlers and no power and are very effective. My standard defensive tactic is to build a door at one point on the wall of the first floor of my building and build a small floor section outside of it. I then extend that around the building with other small floor sections, creating a narrow platform around the building. I garnish the platform liberally with heavy machinegun turrets, usually 8 (1 in each corner, 1 in the centre of each side) to give major firepower in each direction. Obviously, I take into account line of sight and other defences - no point having turrets pointing at a wall a few yards away. I'll also place turrets at entry points and block off points of entry with concrete blocks (structures/wood/floor/the first type of shack foundation) which clip through almost everything and allow you to build impenetrable castle walls. Overkill security usually prevents any attacks at all. I have >150 security on my major settlements, plus complete encircling walls with at least 4 heavy machinegun turrets at the gate. I got called to defend one once, when it was attacked by a band of 14 raiders led by a legendary raider. I fast travelled to it immediately and was just in time to get one shot at the one remaining raider, who was being shot by 3 turrets and 2 settlers at the time. He hadn't made it into the settlement at all and the rest didn't even get that close.
Scrap everything at a new settlement unless you decide not to for roleplay reasons. For example, I haven't scrapped anything at Abernathy Farm because the Abernathy family live there. All I've done there is add lots of security, a water pump and some clean beds under shelter. It would be unreasonable to scrap everything they've done. Hey Connie, is this your favourite chair? Not any more! It's a few bits of cloth and wood that I'll use somewhere else.
Establish provisioners to link settlements. I suggest just 1 or 2 from any one settlement, as they don't contribute to the settlement but do require resources from it (food, water, bed). This lets you share resources between settlements, which makes building so much more convenient and smooths out resource shortages (i.e. a food or water surplus at one settlement will offset a shortage at any other settlement on the chain of provisioned settlements). The link is immediate across all linked settlements, regardless of distance and without any need for direct links. If A is linked to B and B is linked to C, then A is linked to C.
Establish shops. A junk shop will provide a useful source of materials, especially the harder to get stuff like copper. They will also increase the happiness of the settlement and provide you with a continuous income (you may need some unassigned settlers, I'm not sure). It's not much of an income, but it's an income. You probably won't use the other shops other than as places to sell loot, but they contribute to your income and to settler happiness. I've even managed to make Sanctuary nice enough to have Marcy comment that it's not entirely crap! You can also find a few NPC traders who can be persuaded to become settlers and assign them to a L3 shop of the right type, where they will sell some L4 items as well.
I suggest a simple mod that removes (or at least extends) the settlement size limit. That can be done with a simple batch file. You could even do it with just console commands in an unmodded game - the most basic mod for this purpose is just a batch file that you call from the console which executes a few console commands.