am I really that bad at this?

try a medium random map with four (or less) players all set to easy.  maybe give yourself an ally if you need it.  turn pirates off.  and either play entrenchment or the original sins of a solar empire version instead of the diplomacy.  this will help limit the features you need to learn to defeat the ai and give you more time to build fleet and turtle into your planets.

Early in the game, asteroids are good places to grab since they can be cleared quickly and require less investment of time and resources to get them to contribute to your positive income flow. Terran and Desert planets have the potential to become valuable assets (Terran for population and credit income, Desert for logistical space), but also have the heaviest militia forces on them - always at least 1 heavy cruiser, sometimes as many as 3.

Light frigates (Cobalt, Disciple, Ravastra) will help in grabbing those asteroids, but it is a better idea to try to get access to long range frigates (Javelis, Illuminator, Kanrak) as they produce better damage for their cost early on.

YES!  YOU ARE REALLY BAD!

I'm joking.   I consider myself pretty good at RTS games, and sometimes I even get by butt kicked.  (I'm a sore loser to AI too.  Like they take one of my major systems and I rage quit.  don't have that problem against human opponents, but those AI players have no sense of humor)

Yeah, what the the previous posters said. what difficulty are you playing?

As with any RTS, maximizing income vs. outcome.  If you ever have enough resources to build something, do it unless you're saving for something specific.  (in which case you should get THAT the second your resources meet it)

You should tell us what you are *normally* doing, so that we can tell you if it makes sense or no.

Like "I start as <race>, I select <cap> then I use it to <what>, I construct <structure> and produce <what>. Then I make <what lab> and I research <what> ... "
That sort of story.

Are you being sure to purchase the population upgrade for the planets you're taking?  If not you'll end up having little credit income.  It's a common mistake made by many beginners.  Also, try to think, "fleet, fleet, fleet!".  Try to build a large fleet of long range frigates.

What you can do is also play a game and then watch reply to see what AI is doing and try to copy his moves....

From the sounds of it, you weren't expanding at all and were getting beaten by an AI which controlled 90% of the solar system.  You can double your income in the first five minutes of the game if you expand aggressively, and a an experienced player can easily top 100 credits per second within the first hour of the game if the enemy is playing passively.  To put it simply, you will get crushed by a much larger and more powerful opponent if you don't grow your empire.  You don't suck, you just don't know what you're supposed to be doing. 

When the game begins, almost every planet is controlled by neutral militia.  These are not allied with the enemies, and they will attack them as well.  You will need to destroy these militia to capture the planet.  It's very important to scout planets early so you know how many units are defending and how many you need to build to capture them.  Generally your capital ship can handle all but the most powerful of militias. 

how can one build a large fleet and second capital ship and sustain population upgrades for new planets at the same time

i've never been able to do it

the population upgrade is critical.  without it you will be taxed into nothing as you expand.

tech upgrades to increase resourecs and income are also helpful.  one error i have seen several people make is that they increase the fleet limit way too early.  even if you arent using the capacity, the tax is still in effect.  against the ai, only upgrade the fleet capacity after you have finish getting your income in order.

with all settings normal, you can build a colonizer capital ship and a few frigates right at the start.  use these conservatively until you capture a few planets/astroids.  focus on your income after that keeping fleet maintenance taxes low.

everyone will have their own suggestions and techniques, but follow these basic practices and you will have fun and rule the map.

Because of economic constants or time? If its the latter, start using the empire tree to issue all planet based orders. If its the former, well you are probably either not expanding fast enough or are wasting your resources in unnecessary research or something else you should not be focusing on early.

There are some very good replays of pro players how to develop economy. Those are most in bigger matches 3v3 - 5v5 but if you watch them I'm sure you will get some idea how it is done properly.

Of course you will have to be slower since you are building fleet as well but one advice especially against ai is to go full economy (don't purchase any additional fleet supply and sell your cap ship factory as soon as you build that first free cap) and spamm trade ports on all planets ( leave 3 slots empty so you can build military labs a little later on ) and on planet 1 or 2 phase jumps away from enemy build 2 or 3 frigate factories. Sell factory on your HW and build another trade port. And now you can go fully military do research and build ships..... 

PS Don't have time at the moment but if you want i can post some of my replies. I'm not the best in sins but i'm good enough...

What you say is true--it's hard to do it all at one time.  You have to assess what's important and what makes for the best long-term investment.  If you need to fight early, then having a large fleet will be your best investment.  If the enemy is far away or you don't have to fight for some reason, then you can "go eco".

You really need to develop a sense of when you need to "peak" the size of your fleet.  When do you think you will actually end up fighting?  When do you think you will want to pump out lots of ships?  The idea is to be able to maximize the size and power of your fleet at that time.  If your scouts tell you that the enemy is far away then you can focus on economic development.  But, if the enemy is just two jumps away then you need to prepare to fight.

For example, if you don't anticipate that you'll be fighting for 30 minutes and you continuously pump out ships while sacrificing economic development, you might end up having a smaller fleet 30 minutes out than you would have been able to afford had you made some earlier economic investments.  So, to an extent it's all about timing.