I really need help with my Itinerary, please advice

We can't choose how many days you spend in 20 places, but we can give general advice. Make a list of what you want to see/do in each place and make sure that is covered first. Then take any transport issues into consideration. Last, I look at the overall balance of the trip to make sure I have some longer stays of at least four days (for rest as well as practical reasons like doing laundry).
Home base vs. moving depends on how long you like to be in a car per day. I map it all out on Google.

I was confused by this:

Any key places im missing here?? I was planing to spend couple of days
in Sevilla and Cordoba, but Im afraid its not much to do
there??....and maybe way too hot in July ??
On the contrary, there is more to do in those two cities than everything on your list aside form Madrid and Barcelona.

(Originally I was planning to spend couple of days in Sevilla and Cordoba, but Im afraid its not much to do there, and maybe way too hot in July)

Yes, it will be hot in July but there is plenty to do there. I spent 4 days in Seville and just 1 day in Cordoba and didn't see/do everything.

Yeah, take a deep breath and relax, you probably need to prioritize.

Yes, heat will be a problem, the coast helps, but it still will be hot. My first impression is that you have a bunch of coastal towns, many of which are not really quaint Spanish towns, but ex-pat havens, which to be honest, can be fun, but pretty much alike.

I would trim down the movement. Plan at least 3 nights in Madrid, that gives you 2 full days, there are lots of markets and tapas to enjoy, if you want to include museums and the palace and a couple other sights, then add a day.

Once you pick up the car, sure, head to Estepona, from there you can explore other towns on that part of the coast, maybe even up to Gibraltar, again, at least 3 nights.

We enjoy Nerja, will be there for the third time in May, it is just a nice coastal town, decent restaurants and beach, and not much else. Use that as a base, again 3-4nights depending on what you want to see.

Then move up the coast, choose either Xavia or Altea, not familiar with either.

Then to Valencia.

At that point, you are 15 to 20 days into your trip, 3-4 days in Barcelona would work, maybe you can squeeze in Blanes.

I would forget about France, that is another trip, those are further away than you think.

Overall, stay no place less than 3 nights, especially if you plan to daytrip to nearby areas. Four nights would be better some places, Madrid, Valencia and Barcelona warrant at least four usually.

Why in the world are you choosing Marbella, Nerja and Estepona for your beach time? Those might be the lest enticing for me, personally, if I wanted beach time.
For one thing, the Costa del Sol is jammed with tourists, but all beaches will be, so that's kind of moot. Secondly, the beaches in that stretch are not all that nice and there are SO MANY other MUCh nicer places to spend beach time in Spain than the ones you've mentioned. And it will not exactly be cool in Malaga in July.

For gorgeous beaches and more moderate temperatures, would Galicia be of any interest? Glorious region, very green with beautiful beaches and unique architecture, food,, etc.....

Why did you choose Altea and Javea and not, for example, Denia?
And near Barcelona, why Blanes and not the much prettier towns north of the city on the Costa Brava, like Begur or even Tossa?

Who can tell you how many days you need in one place? For example,, I spent 9 nights in Valencia last month, and 5 in Denia, but your interests are probably much different than mine. What is drawing you to Valencia? I hope not the beach!!! It's a great city but I am not getting any idea of your interests besides the Beach..

For me, if I wanted beach time in July, I would look at:

Galicia

North coast of Spain from San Sebastián west to gorgeous beaches of Asturias (car better once you leave San Sebastián and go west

Costa Brava, with car, from Barcelona

Costa de la Luz around Conil, El Palmar de Vejer, etc. Beautiful white sandy beaches, winds cool off temperatures, known for seafood (as are all beach areas!!).

I'm guessing you're at the early stages of planning. I'm similar to you in that I want to see and do it all initially, but realistically can't, so I usually start with google maps just to see where everything is situated to figure out what makes the most sense in order to use as little time as possible for travel. I try not to back track any more than necessary, and I also try to find at least an occasional base where I can settle in for 4-5 days and just do day trips so I can catch my breath.

You only have 25-30 days, which is a lot more than many people get to take a trip, but I'd probably save France for another time. If you're thinking of flying into Madrid then taking a train to Malaga, does it maybe make sense just to fly straight to Malaga and start from there and work your way up? Just an idea.

If I were you (and only you know your interests), I wouldn't skip Seville and Cordoba if I was going to be in southern Spain. I would imagine everywhere will be hot in July anyway. I wore short sleeves in Malaga and Valencia 2 weeks ago, but bought a scarf in Seville because it was colder than I'd expected.

This site is very helpful for honing in on a good itinerary, but keep in mind that everyone has their own bias, so knowing what you want out of a trip and balancing that with the knowledge of those on this forum is probably a good idea.

Good luck with your planning. No matter what you end up doing/seeing, you'll be in Spain which is amazing!

How many days in each spot is your primary questions and no one can answer that, or in fact, they will. So if I tell you 3 nights in Valencia, I am sure someone will chime in with 2 nights or 4 nights. Plus you will get advice on day trips from many of these cities.
All the advice so far is good. You need to do some research. It is always a problem with itineraries with anyone, even experienced travelers and they have to figure out for themselves what timeframes in each place they visit.
You need to do some research and make decisions. It is your trip. Take control. You cannot see and do everything and you will miss some things and perhaps stay longer or less in a place than is ideal for you. It is not the end of the world.